ADDBESS. 35 



the heat retained by the vapours which at that period were diffused in 

 the earth's atmosphere. Indeed, but for the vapour in our atmosphere, a 

 single night would suffice to destroy the whole vegetation of the temperate 

 regions. 



Inspired by a contemplation of Graham Bell's ingenious experi- 

 ments with intermittent beams on solid bodies, Tjmdall took a new 

 and original departure; and regarding the sounds as due to changes 

 of temperature he concluded that the same method would prove applic- 

 able to gases. He thus found himself in possession of a new and 

 independent method of procedure. It need perhaps be hardly added that, 

 when submitted to this new test, his former conclusions on the inter- 

 action of heat and gaseous matter stood their ground. 



The determination of the mechanical equivalent of heat is mainly due 

 to the researches of Mayer and Joule. Mayer, in 1842, pointed out the 

 mechanical equivalent of heat as a fundamental datum to be determined 

 by experiment. Taking the heat produced by the condensation of air as 

 the equivalent of the work done in compressing the air, he obtained a 

 numerical value of the mechanical equivalent of heat. There was, 

 however, in these experiments, one weak point. The matter operated 

 on did not go through a cycle of changes. He assumed that the 

 production of heat was the only effect of the work done in com- 

 pressing the air. Joule had the merit of being the first to meet this 

 possible source of error. He ascertained that a Aveight of 1 lb. would 

 have to fall 772 feet in order to raise the temperature of 1 lb. of water by 

 1° Fahr. Hirn subsequently attacked the problem from the other side, and 

 showed that if all the heat passing through a steam-engine were turned 

 into work, for every degree Fahr. added to the temperature of a pound of 

 water, enough work could be done to raise a weight of 1 lb. to a height of 

 772 feet. The general result is that, though we cannot create energy 

 we may help ourselves to any extent from the great storehouse of nature. 

 Wind and water, the coal-bed and the forest, afford man an inexhaustible 

 supply of available energy. 



It used to be considered that there was an absolute break between 

 the different states of matter. The continuity of the gaseous, liquid, and 

 solid conditions was first demonstrated by Andrews in 1862. 



Oxygen and nitrogen have been liquefied independently and at the 

 same time by Cailletet and Raoul Pictet. Cailletet also succeeded in 

 liquefying air, and soon afterwards hydrogen was liquefied by Pictet 

 under a pressure of 650 atmospheres, and a cold of 170° Cent, below 

 zero. It even became partly solidified, and he assures us that it fell on 

 the floor with ' the shrill noise of metallic hail.' Thus then it was shown 

 experimentally that there are no such things as absolutely permanent gases. 



The kinetic theory of gases, now generally accepted, refers the elasticity 

 of gases to a motion of translation of their molecules, and we are assured 

 that in the case of hydrogen at a temperature of 60° Fahr. tbey move 

 at an average rate of 6,225 feet in a second ; while, as regards their size, 



d2 



