8o SCIENTIFIC APPARATUS. 



and accordingly chemical combination is then regarded as result- 

 ing from the double recomposition of two elemental molecules. 

 Further, it is maintained that the atomic combinations in one and 

 the same element may be, and in many cases are, numerous in 

 kind according to condition, and that, especially, variation in heat 

 intensity, which is supposed to be intermolecular commotion, is 

 competent to determine the numerical aggregation of the atoms 

 constituting the molecule. This notion has been offered to ex- 

 plain the fact that the degree of refrangibility of the light which is 

 absorbed by the vapour of an element depends upon its tempera- 

 ture, and the belief that the stratification of the sun's chromosphere 

 and reversing layer is not in accord with the densities of its con- 

 stituents, under the supposition that the ordinary law of the ex- 

 pansion of vapours by heat holds good. That such atomic aggre- 

 gation when once effected may be very permanent is shown by the 

 allotropism so frequent among the non-metallic elements. 



After the atomic theory was adopted to satisfy the supposed 

 requirements of chemistry, it became, in an extended form, prolific 

 as a means of interpreting the action of other forces. The fact 

 that the expenditure of a given amount of work in overcoming 

 friction always gives rise to the same amount of heat, and the 

 observation that more heat must enter a given volume of a gas at 

 a given temperature to heat it through a given range of tempera- 

 ture when the gas expands and thrusts back the atmosphere, than 

 when it is protected by a rigid shell, led to the notion that heat is 

 the blow given by a moving atom or molecule, and that the force 

 of the blow depends upon the mass of the molecule and its velo- 

 city. In heat-tension or temperature the velocity only is con- 

 cerned, in heat quantity the mass also. The elastic force of air is 

 thus viewed as the sum of the atomic or molecular blows given to 

 the enclosing vessel. Further, the spread of heat by conduction 

 is viewed as being due to the occasional impact of the more rapidly 

 moving atoms or molecules of the hotter upon the more slowly 

 moving ones of the less hot. From the rate at which a gas enters 



