274 GREAT MEN OF SCIENCE 



equivalent of heat, and a sufficiently approximate value of 

 it, are given. The second publication (1845) states every- 

 thing at full length, and in conclusion adduces many appli- 

 cations to living nature; the third publication applies the 

 energy principle to celestial processes. 



The contents of these papers show that Mayer was the 

 first who (already in the year 1842) saw clearly and in the 

 most general way what had gradually been come upon by a 

 number of investigators, in the course of the development of 

 knowledge either in fragments or in the form of questions of 

 fundamental importance, but was still without foundation. 

 We already find in Galileo, Huygens, and Leibniz, questions 

 regarding mechanical work performed by gravity and other 

 forces, and Huygens was the first to recognise the importance 

 of the product of mass into the square of velocity in phe- 

 nomena of motion, for example in elastic collision, where the 

 sum of these products remains unchanged, however com- 

 plicated the phenomena of motion. The product is now of 

 course known as kinetic energy, when divided by two. 



Then came the great difficulties of the question 'what is 

 heat?' which had been illuminated by the experiments of 

 Rumford and Davy, in which heat had appeared without 

 having anywhere disappeared, whereas its quantity had 

 otherwise - in particular in calorimetric measurement - 

 proved to be just as invariable as a quantity of any kind of 

 matter. When the phenomena of the electric current came 

 to be known, the heat of the electric arc and of a wire made to 

 glow by a current again presented a riddle; for no compen- 

 sating cold appeared, excepting only in the case of thermo- 

 elements discovered by Peltier in 1834. As regards Volta's 

 elements, Faraday, as well as Carnot before him, had re- 

 marked that they exhaust themselves in the production of all 

 phenomena of whatever kind, and the cause of this might be 

 supposed to be due to the chemical changes shown by 

 Faraday to accompany invariably the passage of current 



