METEOROLOGY. 411 



altitude of 59° j the number, however, diminished in proportion as the 

 sun got lower. This shows that absorption of solar radiation by car- 

 bonic acid is selective, and that the absorbable wave lengths become more 

 rare the greater the atmospheric layer the rays have already traversed. 

 The author calculates from his experiments the proportion of carbonic 

 acid in the atmosphere, finding it 3.27 in 10,000 parts by volume, a 

 number agreeing so well with results of chemical analysis as to in- 

 dicate that this is a good way of determining the carbonic acid in the 

 atmosphere and its variations, applicable, too, at heights when direct 

 measurements are impossible. (Nature, December 30, 1880, xxiii, p. 

 209.) 



Prof. -William Crookes has published some observations on radiation of 

 heat from thermometers inclosed in air at very low pressure. His results 

 have an important bearing on radiation, conduction, and convection of 

 heat. An accurate thermometer, with pretty open scale, was inclosed 

 in a 1^-inch glass globe, the bulb of the thermometer being in the center 

 and the stem being inclosed in the tube leading from the glass globe to 

 the air-pump. There are two ways in which heat can get from the glass 

 globe to the thermometer: (1) By radiation across the intervening space; 

 (2) By communicating an increase of motion to the molecules of the gas, 

 which carry it to the thermometer. It is quite conceivable that a con- 

 siderable part, especially in the case of heat of low refrangibility, may be 

 transferred by " carriage," (as I will call it to distinguish it from con- 

 vection, which is different,) and yet that we would not perceive much 

 diminution of transference, and consequently much diminution of rate 

 of rise with increased exhaustion, so long as we work with ordinary 

 exhaustions up to 1 millimeter. For if, on the one hand, there are 

 fewer molecules impinging on the warm body (which is adverse to the 

 carriage of heat), yet on the other the mean length of path between 

 collisions is increased, so that the augmented motion ii carried further. 

 The number of steps by which the temperature passes from the warm- 

 er to the cooler body is diminished, and accordingly the value of each 

 step is increased. Hence the increase in the difference of velocity 

 before and after impact may make up for the diminution in the number 

 of molecules impinging. It is therefore conceivable that it may not be 

 till such high exhaustions are reached that the mean length of path 

 between collisions becomes comparable with the diameter of the enclos- 

 ure that further exhaustion produces a notable fall in the rate at which 

 heat is conveyed from the case to the thermometer. 



The above experiments show that there is such notable fall ; a reduc- 

 tion of pressure from 5™ to 2™, produces twice as much fall in the rate 

 as is obtained by the whole exhaustion from 760 millions to 1 million. 

 We may legitimately infer that each additional diminution of a millionth 

 would produce a still greater retardation of cooling, so that in such 

 vacua as exist in planetary space the loss of heat, which in that case 



