either major or minor, corresponding to a speed for each of between thirty-five and forty miles an hour.

HEAT.

On the subject of Heat, Cailletet has further studied the effect of pressure on combustion, the experiments being made up to three hundred atmospheres. He finds that while the luminosity of a flame increases under pressure, the activity of the combustion actually diminishes; the temperature augments, but the oxidation lessens. An alcohol flame, ordinarily so pale, becomes as bright as that of a candle at twenty atmospheres. A candle flame under these conditions gives more light, but the wick soon becomes smoky from imperfect combustion that which is gained on the one side being lost on the other.

Violle has called attention to the thermo-diffusion experiments of Feddersen and Dufour (which are properly such, since the diffusion of a gas through a porous diaphragm causes a rise of temperature on the side of the entering gas, and a difference of temperature on the two sides of such a diaphragm causes a diffusion of gas), in order to explain an experiment of Dufour, in which he used air in different hygrometric states on the two sides of the diaphragm, and observed the diffusion. Violle believes that the true explanation of this result is to be found in Merget's experiments, in which a porous cell, filled with pumice in fragments, and closed by a cork through which a tube passes, the whole being well moistened, develops, when exteriorly heated to a dull red heat, simply from the surface evaporation, a pressure of air in its interior of three atmospheres. Experiments of his own show how extremely sensitive is this apparatus to changes of temperature. The practical importance of these facts is very great. Our clothes, the stones of our houses, the very soil itself, when heated after previous moistening, act exactly like the apparatus of Merget, with an activity truly surprising. In animals this gaseous movement plays its part in respiration; but in plants, especially in aquatic plants, it is seen in full activity, Nelumbium speciosum, for example, throwing from its stomata half a liter of air per minute, solely through this action going on in its leaves.

Berthelot has published an important research, in which he

has studied the thermal changes produced when acids or alkalies are dissolved in water, with the expectation of solving the question of hydratation. He has also given a description in a subsequent memoir of the various pieces of apparatus which he has employed in his calorimetrical experiments. These are, a helicoidal agitator for mixing the water of the calorimeter, an écraseur for crushing salts and other solids in liquids, a distilling apparatus, with worm and receiver, for effecting reactions out of contact with water, an apparatus for measuring the heat of solution at elevated temperatures, a closed apparatus for the reaction of nitrogen dioxide on oxygen, and an apparatus for decomposing ammonium nitrite by heat.

Thomsen has made another series of investigations in thermo-chemistry, in which the heat of combination of manganese, zinc, cadmium, and iron has been determined. Combining these results with previous ones, it appears that for the nine metals which decompose hydrochloric acid with evolution of hydrogen, the heat of combination for every molecule of hydrogen thus evolved is, for lithium, 125,860 calories; for potassium, 123,700; for sodium, 114,380; for magnesium, 108,290; for aluminum, 79,880; for manganese, 49,360; for zinc, 34,200; for iron, 21,310; and for cadmium, 17,610 calories.

Boisbaudran has shown that a remarkable inequality of action is exerted by a given supersaturated solution upon different isomorphous bodies. A perfectly regular crystal of potassio-chrome alum, placed in a slightly supersaturated solution of ammonio-alumina alum-which had been rendered basic, so as to crystallize in cubes-was soon covered with a white octohedric envelope showing cubic facets. After a longer time the cubic facets had increased considerably, but the distances between opposite solid angles of the octohedron remained unaltered. Hence the author concludes that the solution must have been supersaturated relatively to the octohedral faces of the ammonio-alumina alum, but not relatively to the cubic faces of the same alum. In general it appears that in the phenomena of solution and crystallization, the molecular volume, the density, the relative arrangement of the similar or dissimilar atoms in the molecule, and all other causes of dissimilarity, possess their special influences.

Indeed, it may be said that two bodies not absolutely identical never exhibit strictly the same physical or chemical reactions, however closely they may in certain particulars resemble each other.

Pfaundler confirms the unequal solubility of different faces of the same crystal, remarked by Lecoq de Boisbaudran, and calls attention to his theoretical explanation of it, first published in 1869. He concludes that those faces of a crystal which possess favorable conditions for resisting the impact of the moving molecules are preserved and grow at the expense of the others. "Thus," he says, "the principle laid down by Darwin is applicable also in the world of molecules. Those forms and combinations which possess the most favorable conditions of existence are the ones which are preserved."

Boisbaudran has also shown that very low temperatures may be produced by means of the ammonia ice-machine of Carré by taking suitable precautions. If during the cooling the heater be surrounded with ice-water, or, still better, with a freezing-mixture, it is possible to obtain, even with a machine holding only half a liter, the rapid solidification of several kilogrammes of mercury. After the freezing of nearly five kilogrammes of this metal in a solid cylinder, the temperature within was found to be 48°. If ice and salt be added to the water in which the condenser is placed during the heating, it is not necessary to raise the temperature of the heater so high by ten or fifteen degrees.

Guthrie has given a curious paper upon hydrates (or hydrated salts) formed at a low temperature, which he calls cryohydrates. He shows, contrary to the generally received opinion, that the minimum temperature attainable by mixing ice with a salt is very independent of the ratio of the two, and of their temperature, and of the state of division of the ice. The temperature of a mixture of ice and a salt is as constant and precise as the melting-point of ice. He observes that the cryohydrates of the nine salts which potassiam, sodium, and ammonium severally form with chlorine, bromine, and iodine, are formed at temperatures ranging from -28° to -11°. Thirty-five salts were examined in this way, and it was found that the temperature at which the cryohydrate is formed is precisely that obtained by mixing the given salt with ice. In a subsequent paper he gives addi

tional experiments upon salt solutions and attached water. He assigns the name cryogen to an appliance for obtaining a temperature below 0° C., and cryohydrate to the substance. produced by the union of water with a body, this hydrate being capable of existence only below 0° C. He finds that of cryogens the best is a mixture of sodium bromide with three to six times its weight of ice finely divided, the temperature produced being -28° C. From an extended series of experiments, he concludes that "of similar salts, the one which produces the greatest cold when used in a freezing mixture unites as a cryohydrate with the fewest molecules of water." And again, "The temperature at which the cryohydrate is formed is the same as the temperature of the corresponding freezing mixture." Of special interest is the cryohydrate of ethyl alcohol, which is produced whenever a dilute alcohol is exposed to a temperature of -34° C., and has four water molecules united to one of alcohol. It separates from the liquid in crystals. Ether also forms a cryohydrate, solidifying at -2° C., and consisting of one ninth of ether. If the experiment be made in a long test-tube, the long candle-like mass, when removed, placed upright on a plate, and lighted, burns with a non-luminous flame, the heat being consumed in melting the ice.

Chaumont has experimentally investigated the question of ventilation, so far, at least, as the amount of air necessary for health is concerned. His determinations were made on the air of barracks, of prisons, and of hospitals; and he concludes from them that 85 cubic meters (3000 cubic feet) of air per head per hour is necessary in health, in ordinary diseases one third more than this, and in serious diseases and epidemics even more still.

Gernez has made an exhaustive research into the phenomena attending ebullition. His paper opens with a long historical note upon this subject. Then follows his own results, in which he studied (1) liquids heated in contact with solids, (2) within other liquids, and (3) the ebullition developed by mechanical action. He maintains that ebullition is an evaporation into some gaseous atmosphere contained within the liquid.

Troost and Hautefeuille have made a calorimetrical investigation on iron and manganese silicides. They conclude,

first, that silicon in combining with manganese evolves considerable heat, and hence that the compound thus formed is very stable-a fact already proved for carbon. Second, that the similarity of these two substances, carbon and silicon, appears also when their action on iron is considered; they both act as if they were dissolved in the metal.

Kundt and Warburg have obtained an interesting result in investigating the specific heat of mercury vapor. On the kinetic molecular theory of Clausius, the quotient of the specific heat of a gas at constant pressure, divided by the specific heat of the same gas at constant volume, should be 1.67, while, in fact, for most gases this quotient is only 1.405. Clausius explains this by the fact that molecules are not material points, but are composed of atoms; and only in a monatomic gas would there be a correspondence with theory. The molecule of mercury is shown by its vapor density to be monatomic; and it is now found by experiment that in the case of this vapor the above quotient is actually 1.67. Hence a molecule of mercury, so far as its theoretical and mechanical properties are concerned, acts like a material point.

Desains has continued his researches upon solar radiation, and has determined the quantity of heat received per minute at Paris by one square centimeter of the earth's surface placed normal to the direction of the rays during an entire year. The maximum was on June 22, when the amount received was 1.29 units, and the minimum on January 30, the amount being one unit. He finds also that the proportion of the solar rays transmitted by a layer of water eight millimeters thick reached its maximum on July 4, being 0.71, and its minimum on April 25, being 0.63,

Mayer has proposed a simple mode of obtaining thermographs of the isothermals of the solar disk by the use of Meusel's double iodide. Thin paper, smoked on one side, is covered on the other with the iodide, and is exposed to the sun's image, formed by a telescopic object-glass, the aperture being at first only that necessary to give the smallest arca of blackened iodide with a sharp contour. This he calls the area of maximum temperature. On enlarging the aperture, the black area gradually extends, forming a series of new isothermal lines with the successive enlargements. Some interesting conclusions have already been reached, and it is