PHYSICS. 263 



bon dioxide for radiant heat, with especial reference to the effect of this 

 gas in the atmosphere. Usiug a gas Haine and a glass cylinder, 

 he found that a layer of the gas 214'"°» thick allowed 94.8 per cent, 

 of the radiation to pass; 530"^"^ allowed 93.8 per cent.; and 917'""^ 89 

 per cent. The sun's rays were proved to undergo considerable weak- 

 ening in their passage through carbon dioxide ; a layer of the gas one 

 meter thick absorbing 13 per cent, when the sun's altitude was 59°, 

 diminishing as tlie sun got lower. The absorption of solar radiation 

 by carbon dioxide is selective therefore, the absorbable wave-lengths 

 becoming rarer as the thickness of the atmosphere already traversed 

 increases. From his experiments. Lecher calculates the proportion of 

 carbon dioxide in the atmosphere to be 3.27 parts in 10,000 by volume. 

 The agreement of this value with that obtained by direct chemical 

 methods suggests that absorption is a good method for its rapid deter- 

 mination in the air. — {Mature, xxiii, 209, December, 1880.) 



3. Specific heat. 



Mile. Stamo has publislied as an inaugural dissertation at Zurich 

 some researches upon the specific heat of water. In the first set of experi- 

 ments a brass calorimeter was used, filled to within one centimeter of 

 the top with copper turnings loosely packed. Into this the water, 

 heated gradually in a copper vessel placed in an oil bath, was poured. 

 The high conducting power of the copper turnings rendered it unneces- 

 sary to agitate the liquid. The author having determined in advance, 

 as exactly as possible, the specific heat of copper as compared with 

 water at ordinary temperatures, the comparison could easily be made 

 with the water heated to GiP or 80^. The mean of the coefficient of va- 

 riation of the specific heat of water is 0.0012550. In the second set 

 of experiments, the same electric current was passed through wires of 

 platinum, or of German silver, about 3""" long, rolled on rubber frames 

 and immersed in calorimeters full of water, one being surrounded 

 with ice, and the other with water at a constant temperature t. From 

 the heating observed, the specific heats at 0° and at P were calculated. 

 In this way the variation coefficient was found to be 0.000859, a value 

 of the same order as in the first case. — (<7. Phi/s., ix, 138, April, 1880.) 



Witz has examined the cooling jiower of air at high pressures with a 

 view to verify the exactitude of the law of Dulong and Petit, according 

 to which the heat removed at each instant from a body by any gas sur- 

 rounding it depends only upon the excess of temperature of the body 

 above the surrounding medium, and upon the pressure of the gas. The 

 apparatus consisted of a cast-iron cylinder of 13 liters capacity, strong 

 enough to sustain a pressure of 10 atmosj^heres, lampblacked on its in- 

 terior. Within this was placed a cylindrical cup of thin iron, 51""" long 

 and 27™"" in diameter, surmounted by a tube 8™*" diameter, by which it 

 is adjusted to the center of the large cylinder. The cup holds 200 grams 

 of mercury, and the stem of a thermometer is j)lunged into it. The 



