lecturing, teaching, advising, for everyone in Irvine sought counsel of its "living oracle," and the house of this dissenting minister was a kind of priestly confessional for all who were in trouble. All counted on his tender sympathy, and all confided in his insight and wisdom. No one was more entirely loved and trusted, and probably he knew more family secrets than the most approved family solicitor. A bachelor, living with a devoted sister, yet women of all kinds, married and single, brought their burden of cares to him, for he was a natural-born priest, without a shadow of the "craft." Trust came to him: he never sought it. He was at home in the human heart; but he never seemed to probe it. He did not handle cases of conscience in his pulpit, yet people brought their doubts and scruples to get direction from him, and I doubt not that he helped them, for his heart felt with them. So the years passed amid the love and the honour of all who knew him, till, in 1871, he was laid down by an attack of pleurisy and effusion, which brought him nigh the brink of the grave. For many days his life was despaired of, and the only hope rested on a very delicate and critical operation. Happily it was successful so far, and he lived for a good many years after, but his work as minister of Irvine was ended. Henceforth he only preached occasionally at distant intervals, mostly for friends. His many admirers cheerfully made up a purse to provide for his remaining years, and he left Scotland to seek a milder air in Italy.

Robertson's artistic instincts had always craved for that land of sunshine and beauty, and now they could be gratified to the full. Ere long his health in a great measure was restored, so far at least that he could make a home in Florence in spite of the Tramontana. For Florence was more to him than Rome, and ere long he pervaded the Tuscan capital almost as he had done Irvine. At least, few English-speaking folk went there who did not see it through his eyes, for he had studied and knew its treasures as only Ruskin among living men had done. Its architecture, its paintings, its sculpture, the lives of its great men, the story of its rise and decay, its religious life and its common life, both past and present he soon became familiar with all, and discoursed of them by the hour as one who loved them, and brought all the wealth of a vivid imagination to illustrate them. And love them he did, in spite of his staunch Presbyterian Protestantism which remained for all that as staunch as ever to the end.

He formed many warm friendships among the higher class of Italian priests, and often spoke to me in after years of the joy it gave him to find so much Christian fellowship with them. He could understand, he said, how Leighton often left the Presbytery to get his heart refreshed in a monastery like La Trappe. At times, he even went so far as to join in services which would have made some of his brethren stare and gasp. On one occasion, e.g. driving with his sister into a town, they met a procession marching to the shrine of their patron saint, headed by the priests, and chanting one of the old Latin hymns. Whereupon he ordered the carriage to stop, and jumped out and joined them, singing with his deep musical bass the grand old strain, as probably no one else was able to do. He was passionately fond of music, especially of the ancient ecclesiastical chants and plain-songs, and would sit at his chamber organ dreaming over them far into the night. I doubt not it was this feeling-essentially artistic, not religious-which led him, as the procession came up with banners flying and boy-voices piping the hymn which he had probably often sung to himself, to take his place in the throng, and give a more musical as well as a more spiritual voice to its sacred song. He could not help himself. He was like Saul among the prophets, and must needs sing with them, only he was the real prophet, and the procession were probably rather a lot of Sauls. And yet, who knows?

Of all the books which I often hoped he might be persuaded to write, the one which latterly I urged most strongly on him was a work on Religious Art, and there were times when I fancied he might be got to do it. I tried hard to persuade him to prepare a couple of lectures on the subject for the Philosophical Institution of Edinburgh, which, even if they were not written out, might by means of the reporter be made the nucleus of a more complete work in due time. For without pretending myself to have any knowledge on the subject, or to say whether his ideas were sound or not, it was clear that he had ideas on the subject, very many, and full of interest and of beauty. He had seen and pondered and sought the meaning of Florentine art as no one I ever met had done, and he had compared it with the current of Italian literature and history from Giotto and Dante down to Raphael and Vasari. The colours, the faces, the postures of almost every picture were familiar to him, and he traced a meaning through them all, and an historical relation to the spiritual decay of the people. I can

number of accidents of this class. What seems to be required is a more constant inspection of the workingplaces; the maintenance, in places convenient to the workmen, of an ample supply of timber for propping up the roof; the proper training of the miner as to the best mode of protecting his working-place; the exercise of greater care on his part in watch ing the roof, sides, and face; the introduction of arrangements with the workmen which will make it their interest not to avoid the labour of putting up the necessary timber, &c., for their proper protection; and the employment of special workmen to look after the timbering and the main-ways, and the drawing of the timber from the disused working-places. There is no question also that many of the accidents which are classed together as "miscellaneous" might be obviated by improved discipline, and by the exercise of greater care on the part of those who are employed on the engine-planes and other roadways.

But as regards casualties from explosions, the case is somewhat different. We have here to do with an enemy which is always, so to say, on our flank, which sometimes

does its work insidiously, and at other times by sudden onslaught, and which can only be successfully met by unceasing vigilance, a trained intelligence, and scientific knowledge.

The causes of colliery explosions have seemed at times inscrutable, but, thanks to the labours of the Royal Commissioners, whose report has already been referred to, and to the work of mining engineers and colliery managers in this country and on the continent, we are gradually dispelling the mystery. It is the purpose of this paper to explain what we now know concerning the origin, in general, of these catastrophes, and to indicate how we may hope, in the light of this knowledge, to lessen the frequency of their occur

rence.

In the course of the chemical changes accompanying the transformation of vegetable matter into coal there is produced, in greater or less abundance -depending upon conditions which we need not here go into a gaseous compound of carbon and hydrogen, commonly known as marsh gas. It is so named because it is to be met with in marshy places as a product of the decay of vegetable matter in contact with water. This gas was thought by the older chemists to be identical with the inflammable air formed by the solution of certain metals, as, for example, iron or zinc, in the common acids. The two gases were shown to be distinct by the Italian physicist Volta and by our countryman John Dalton, who pointed out that in the act of burning or by explosion with air marsh gas forms carbonic acid in addition to water, whereas hydrogen-the gas evolved on the solution of metals-under the same conditions gives rise to water only. Marsh gas, however, resembles hydrogen in being much lighter than air, and in being colourless, tasteless, and odourless. When it is mixed with air in due proportion the mixture, if heated by contact with a flame or in other ways to a sufficiently high temperature, gives rise to an explosion, the violence of which depends upon the amount of the admixed air. The most violent explosion is given with an admixture of from nine to ten volumes of air, but air containing only

one-twentieth of its volume of marsh gas is still highly explosive.

Every gas which has the power of combining with oxygen to form a flame, or, in other words, which is capable of burning in the air, needs to have its temperature raised to a certain point before it will ignite. There are certain gases which take fire spontane ously when they come in contact with the air; this means that their ignition temperature is the ordinary temperature of the air. There are other gases which will ignite at the temperature of boiling water. Hydrogen ignites at a low red heat; marsh gas, on the other hand, requires a much higher temperature to bring about its ignition-a redhot poker, which instantly determines the explosive combination of a mixture of hydrogen and air, may be thrust with impunity into a mixture of marsh gas and air. This peculiarity of marsh gas has an important bearing upon the theory of the safety lamp.

Now the fire-damp of the coal-miner consists mainly of marsh gas associated with more or less carbonic acid, or choke-damp, and nitrogen gas. It should be noted that relatively small quantities of the last-named gases greatly affect both the explosive violence of the fire-damp and the amount of air determining the explosive limit. There are certain other conditions which modify the violence of the explosion by influencing the temperature of the flame and the increase of pressure at the moment of chemical change, but as their consideration hardly affects the general question it is unnecessary to dwell upon them now.

Coal has been worked in this country since the time of the Normans; but it was only in the beginning of the seventeenth century that explosions in collieries appear to have been heard of. Even then they were seldom fatal. One which occurred at Mostyn, on the Dee, in 1676, and which killed a man and blew off the winding-drum at the top of the pit, was apparently so novel an event as to be thought worthy of description in the "Philosophical Transactions of the Royal Society." This comparative infrequency of explosions in the early workings is readily accounted for by the mode in which coal was got at that time. The pits were very shallow; indeed, at the beginning of the eighteenth century no pit had reached a greater depth than sixty fathoms; commonly they were not more than from twenty to thirty fathoms deep. To-day some of our pits are half a mile in depth; the Ashton Moss pit at Audenshaw, for example, is close upon four hundred

and fifty fathoms deep. Moreover, the old workings did not extend to any considerable distance from the shafts. In fact, in the early days of coal-getting, the miners were more hindered by water and choke-damp than by explosive gas. Choke-damp must, indeed, have been a sore trouble, if we may judge from the old-fashioned method of bring ing round asphyxiated colliers. The remedy, we are told, "was to dig a hole in the earth and lay them on their bellies with their mouths in it; if that fail they tun them full of good ale; but if that fail they conclude them desperate."

It was only towards the beginning of the last century that fire-damp became really formidable, and as the pits increased in depth the evil became more and more seriously felt Itwas quickly recognised that the best method of dealing with the gas was to sweep it out of the workings by a vigorous air-current; but this, in the early days of coal-getting, was not always practicable. The old proverb that "a prudent miner minds the wind,” had its origin in the days when the ventilation of the mines was solely dependent on the difference between the temperatures of the air in the pit and above ground. When the atmosphere was stagnant, or when the workings were at too great a distance from the shaft, the only method of preventing the accumulation of the gas was to fire it from time to time. The "fireman," covered with sackcloth saturated with water, crept along the ground, inch by inch, towards the spot where the firedamp lurked, holding out before him a long pole carrying a couple of lighted candles. These he cautiously pushed towards the roof, and as the gas ignited he pressed his face to the earth to escape the scorching flame. As the pits were deepened and the workings extended, this method, at all times dangerous, became at length impracticable, and many collieries had to be abandoned owing to the impossibility of working in them with naked lamps or candles. About the middle of the eighteenth century an ingenious mine-manager named Spedding invented the steel-mill, in which a disc of steel is caused to revolve rapidly against a piece of flint. It was by the feeble radiance of the shower of sparks thus caused that the work of the miner could alone be carried on in a so-called "fiery" pit. The action of the instrument was, however, very uncertain, and many ignitions of gas were traceable to its use.

The atmosphere of every coal-mine probably contains more or less marsh gas,