those gases like hydrogen, oxygen, 

 nitrogen which are more " perma- 

 nent," i.e. more difficult to 

 liquefy. 



By degrees the use of the 

 hydrogen or nitrogen thermometer 

 as a standard of comparison for 

 all liquid-in-glass thermometers 

 became the accepted practice in 

 careful research, and the final 

 touch was added by Lord Kelvin 

 (then William Thomson) in 1848, 

 when he established on purely 

 theoretical grounds an "absolute 

 thermodynamic " scale of tem- 

 perature which is independent of 

 the particular properties of any 

 particular substance, and in col- 

 laboration with Joule of Man- 

 chester carried out a famous series 

 of experiments to determine the 

 slight deviations between the indi- 

 cations of a " gas " thermometer 

 and the " absolute scale." 



The Platinum Thermometer 



This settling of the scale of 

 measurement is quite apart from 

 the great practical development 

 which has taken place in recent 

 years in the construction of ther- 

 mometers for special purposes, 

 such as measurement of very low 

 or veiy high temperatures. Mainly 

 owing to the labours of H. L. Cal- 

 lendar the "platinum" thermo- 

 meter has become an instrument 

 of great precision for such ex- 

 tremes as liquid gases and furnaces. 

 In this type alteration of tempera- 

 ture is measured by the variation 

 experienced in the electric resist- 

 ance of a wire of pure platinum 

 mounted and insulated on a mica 

 frame, protected in a tube of por- 

 celain, and connected by suitable 

 leads to apparatus for accurate 

 determination of resistance. By in- 

 serting large porcelain test-tubes in 

 furnaces with their open ends just 

 protruding through the wall of the 

 furnace, and measuring the amount 

 of radiation proceeding from this 

 opening, great precision has been 

 introduced into furnace ther- 

 mometry. In these " radiation 

 pyrometers " use is made of 

 Stefan's law that the amount of 

 radiation emitted from such a 

 " full radiator " as this tube 

 varies according to the fourth 

 power of the temperature as shown 

 on the absolute scale. 



We must be careful to free our 

 minds from any confusion between 

 the famous discussions as to the 

 nature of heat, and the experi- 

 mental work carried out for the 

 purpose of measuring heat. Even 

 at a time when views were enter- 

 tained concerning the nature of 

 heat, which we now regard as 

 quite inadequate, the question of 

 its measurement had advanced a 

 considerable distance along right 



3901 



lines. The early attempts to utilise 

 the mechanical power of steam in 

 Britain were made in the eighteenth 

 century, and James Watt received 

 great assistance from Joseph 

 Black, of Edinburgh, who was the 

 first to elucidate the ideas of 

 "specific heat" and "latent heat." 

 Specific Heats 



In modern units we say that 1 

 " gram-calorie " of heat" is re- 

 quired to raise the temperature of 

 1 gram of water through 1 centi- 

 grade degree, and for other masses 

 and ranges of temperature the 

 amount of heat is in proportion 

 to the product of the two numbers 

 involved. What Black discovered 

 was that other materials had their 

 specific amount of heat for similar 

 changes, different for each sub- 

 stance. Thus, copper requires 

 about 1/11 of a calorie per gram 

 per degree, iron about 1/9, mer- 

 cury 1/30, ice 1/2, turpentine 1/2, 

 etc. Such numbers are referred to 

 as " specific heats " of copper, iron, 

 etc., and a notable fact is the very 

 large "capacity for heat" enjoyed 

 by water in comparison with nearly 

 all other substances, especially the 

 materials of the earth's crust. 



Black also discovered what is, 

 in deference to historic tradi- 

 tion, still called "latent heat " ; 

 i.e. the fact that when a body 

 changes state from solid to 

 liquid or from liquid to vapour, 

 a considerable quantity of heat 

 is required to effect this change of 

 state even without any change of 

 temperature. Thus, the latent 

 heat of fusion (melting) of ice at 

 C. is 80 calories per gram 

 melted ; of tin at 231 C., 14 calories ; 

 of silver at 1,000 C., 21 ; of mer- 

 cury at 39'5 C., 3, etc. ; the latent 

 heat of vaporisation of water at 

 100 C. is 537 calories per gram 

 vaporised ; of ethyl alcohol at 78 

 C., 210 calories ; of turpentine at 

 156 C., 69 calories, etc. 



Joseph Black's Latent Heats 



Similarly definite quantities of 

 heat are involved in chemical 

 changes as distinct from physical, 

 and we speak of " heat of combus- 

 tion," " heat of reaction," " heat 

 of solution." In fact, in Black's 

 mind there was little difference in 

 nature between such heats and his 

 " latent heats." He shared the 

 view, common in his time, that 

 heat was actually a subtle and 

 highly elastic fluid material with 

 different degrees of affinity for 

 ordinary matter and also endowed 

 with the property of self -repulsion 

 (an idea common then and now as 

 regards " positive electricity " or 

 " negative electricity "). Black 

 assumed that the apparent dis- 

 appearance of the heat (since it 

 produced no " sensible " change 



HEAT 



in the temperature) was due to a 

 quasi-chemical combination be- 

 tween the " caloric " (as the hypo- 

 thetical heat fluid was called) and 

 the particles of the melting or 

 vaporising substance, so that 

 water was " ice cum caloric " and 

 steam was " water cum caloric "- 

 i.e. the caloric was latent in the 

 water and in the steam. 



It is generally believed that the 

 modern view as to the nature of 

 heat arose first in the early 19th 

 century. The truth is the belief 

 that heat is a mode of motion is to 

 be found in the works of Descartes, 

 Amontons, Boyle, Francis Bacon, 

 Hooke, and Newton. The theory at 

 that time rested on very slender 

 evidence, so perhaps it is not sur- 

 prising that the 18th century phil- 

 osophers abandoned it in favour 

 of the material hypothesis ; and, 

 indeed, so long as we exclude from 

 consideration the production of 

 heat by friction and percussion, the 

 caloric theory serves as a very 

 adequate theory for thermal phe- 

 nomena. But the literature of the 

 18th century teems with contro- 

 versy on the subject, and the often 

 ingenious attempts of the " cal- 

 orists " to evade the difficulties of 

 frictional heat. The decisive blows 

 at the caloric theory were struck 

 by Benjamin Thompson and Hum- 

 phry Davy. 



Davy's Ice Experiment 



In 1798 Rumford pointed out 

 that in boring cannon out of solid 

 metal the action of the borer 

 poured out heat unlimitedly. " It 

 is hardly necessary to add," he 

 wrote, " that anything which any 

 insulated body can continue to 

 furnish without limitation, cannot 

 possibly be a material substance." 

 Sir Humphry Davy melted ice by 

 rubbing two pieces together by a 

 mechanism in a vacuum. This con- 

 troverted directly the view that 

 caloric was squeezed out of the 

 pores of a body or torn from com- 

 bination with its particles by 

 rubbing (this was the calorist's ex- 

 planation of frictional heat) ; for, 

 as everyone admitted, heat had to 

 be communicated to ice and not 

 " torn from " it, to melt it. 



The famous experiments of J. P. 

 Joule settled the matter finally. 

 Evolving heat by friction of 

 paddles in water, he measured the 

 heat yielded and compared it in 

 every case with the work required 

 to maintain the paddles in motion, 

 discovering that 1 pound-calorie 

 (heat required to raise 1 pound of 

 water through 1 centigrade degree) 

 was produced by the expenditure of 

 approximately 1,400 foot-pounds 

 of work. These experiments re- 

 peated by several other workers 

 under varying conditions form the 



