Dr. J. R. Mayer on the Mechanical Equivalent of Heat. 497 



and we thus arrive at the pregnant discovery that, at a height 

 equal to the earth's semidiameter, the distance fallen through 

 and the final velocity, for the first second, is four times less than 

 on the surface of the earth. 



In order to pursue our inquiry, let us now return to the objects 

 which immediately surround us. From the earliest times, the 

 phenomena of combustion must have claimed in an especial 

 degree the attention of mankind. In order to explain them, the 

 ancients, in accordance with the method of their naturalistic 

 philosophy, put forward a peculiar upward-striving element of 

 Fire, which in conjunction with, and in opposition to, Air, 

 Water, and Earth, constituted all that existed. The necessary 

 consequence of this theory, which they discussed with the most 

 acute sagacity, was, that in regard to the phenomena in question 

 and all that related to them, they remained in complete 

 ignorance. 



Here, again, it is quantitative determinations, it is numbers 

 alone, which put the Ariadne's clue in our hand. If we want 

 to know what goes on during the phenomena of combustion, we 

 must weigh the substances before and after they are burned ; 

 and here the knowledge we have already acquired of the weight 

 of gaseous bodies comes to our aid. We then find that, in every 

 case of combustion, substances which previously existed in a 

 separate state enter into an intimate union with each other, and 

 that the total weight of the substances remains the same both 

 before and after the combination. We thus come to know the 

 different bodies in their separate and in their combined states, 

 and learn how to transform them from one of these states into the 

 other ; we learn, for instance, that water is composed of two kinds 

 of air which combine with each other in the proportion of 1 : 8. 

 An entrance into chemical science is thus opened to us, and 

 the numerical laws which regulate the combinations of matter 

 {die Stbchiometrie) hang like ripe fruit before us. 



As we proceed further in our investigations, we find that in 

 all chemical operations — combinations as well as decomposi- 

 tions — changes of temperature occur, which, according to the 

 varying circumstances of different cases, are of all degrees of 

 intensity, from the most violent heat downwards. We have 

 measured quantitatively the heat developed, or counted the 

 number of heat-units, and have so come into possession of the 

 law of the evolution of heat in chemical processes. 



We have long known, however, that in innumerable cases heat 

 makes its appearance where no chemical action is going on ; for 

 instance, whenever there is friction, when unelastic bodies strike 

 one another, and when aeriform bodies are compressed. 



What then takes place when heat is evolved in such ways as these ? 



