HEAT. 



495 



everywhere ; and, so long as there is a univer- 

 sal material vehicle for motion, the conception 

 of a hypothetical ether is superfluous. But it is 

 replied that, by the term ether, is meant this uni- 

 versal material, something capable of motion, 

 and assumed to possess certain definite proper- 

 ties. Some such conception is necessary at the 

 present time, in order to express those systems 

 of movement in which the various forces 

 consist. 



As thermometric heat, or the heat of con- 

 duction, is a motion of the constituent atoms 

 of bodies, so radiant heat, or that which darts 

 forward rapidly in straight lines, is a move- 

 ment of the ether. Light is no longer the 

 shooting of corpuscular particles ; it is a cer- 

 tain rate of undulation of the ethereal medium 

 it is motion. The different colors result 

 f-om different rates of undulation. The va- 

 rious actinic, or chemical rays, are due to the 

 same cause, and thus there is seen to be a close 

 correlation between the radiant forces ; they are 

 all but modes of motion. The vibrations of the 

 atoms may impart motion to the ether as it oc- 

 curs in the radiation of every heated body ; 

 and, conversely, the undulations of the ether 

 may be spent in setting the particles of bodies 

 in motion, and thus bodies are warmed by radi- 

 ation. 



The most recent and important step in the 

 progress of thermotic science has been made 

 by Prof. Tyndall, and consists of an analysis of 

 the relations of radiant heat to gaseous bodies, 

 and especially to water vapor. We condense 

 from the new edition of Youmans' Chemistry, 

 in which the recent views are fully developed, 

 a statement of the principles involved in this 

 subject. An opaque body destroys the lumin- 

 ous waves which fall upon it ; while a trans- 

 parent one permits them to glide through be- 

 tween the atoms without interference. But 

 there are bodies which destroy some of the 

 waves and allow others to pass. If a piece of 

 red glass be placed between the prism and the 

 spectrum it stops the blue rays and transmits 

 only the red that is, it cuts down the more 

 minute waves and gives passage only to the 

 larger. If blue glass be used there is a reverse 

 effect, the red rays being extinguished and the 

 blue alone transmitted. Both glasses are 

 transparent, yet, if placed together in the path 

 of the rays, they are as opaque as a plate of 

 iron, each destroying what the other transmits. 



This is also the case with the heat rays ; they 

 are of different kinds like the colors of light, 

 and are arrested and transmitted differently 

 by different substances. Bock salt is the most 

 perfect diathermic body ; that is, it allows all 

 the heat rays, those from the sun and from the 

 hand to pass through with equal freedom. 

 Glass and a thin film of water will absorb or 

 arrest the dark or obscure radiations, while 

 they will pass luminous heat or those radia- 

 tions which come from a luminous source. It 

 is well known that the sunbeam is a bundle 

 of heterogeneous radiations, and that the prism 



spreads them out into a spectrum, thermal at 

 one end, chemical at the other, and' luminous 

 in the centre. The same thing holds true of 

 all sources of heat, luminous and obscure 

 they emit rays of different qualities. When 

 the mixed rays from any source are passed 

 through a plate, a certain portion of them is 

 stopped, and another portion transmitted. But 

 if the rays that are passed are made to fall upon 

 a second similar plate, a much larger portion 

 will be transmitted than went through the first 

 the first plate sifted the ray, and the purified 

 beam is better fitted to penetrate another sim- 

 ilar plate. This principle explains the fact that 

 glass readily transmits solar heat, while it stops 

 the heat from a red-hot cannon ball in large 

 quantities. The rays of the sun in coming 

 through the atmosphere are strained of those 

 rays which would be stopped by glass, so that 

 the altered beam passes our windows without 

 loss. 



Tyndall's apparatus for investigating the in- 

 fluence of gases upon radiant heat, consisted 

 of a long glass tube three inches in diameter, 

 closed air tight at either end by caps of pure 

 rock salt, and connected with apparatus so as 

 to be exhausted and filled with various gases 

 at pleasure. At one end of the tube was placed 

 his source of heat, a blackened canister of hot 

 water, and at the other end a thermo-electric 

 pile the most delicate instrument for measur- 

 ing or detecting heat. By this machine, con- 

 trolled so carefully as to secure the utmost 

 precaution against error, Tyndall exposed vari- 

 ous gaseous bodies to the dark thermal radi- 

 ations. Purified air was found to arrest none 

 or an exceedingly minute proportion of the 

 rays ; while pure oxygen, hydrogen, and nitro- 

 gen behave in a similar manner, being almost 

 neutral. But when compound gases were in- 

 troduced, there was a remarkable effect : olefi- 

 ant gas, which is just as transparent as air, 

 arrests 80 p. c. of the rays of heat Pure trans- 

 parent ammonia is still more impenetrable and 

 stops the heat as light would be stopped if the 

 cylinder were filled with ink. The same ef- 

 fect is produced if only a small proportion of 

 these gases is mingled with the air of the 

 cylinder. 



In this manner, invisible gases become the 

 means of sounding the atomic constitution of 

 bodies. While heat rays pass through common 

 oxygen with out being intercepted, ozone, which 

 is but another form of oxygen, arrests a large 

 proportion of it like compound gases ; we there- 

 fore infer that its atoms are arranged in groups 

 or complex molecules. When aqueous vapor 

 was introduced into the tube, it was found to be 

 highly opaque to the dark radiations. Where 

 the atmospheric gases arrest one ray of obscure 

 heat, the small proportion of watery vapor con- 

 tained in the air strikes down sixty or seventy 

 rays. The consequences of this fact are in 

 every way of the highest importance in the 

 economy of nature. Luminous heat from the 

 sun penetrates the air, and falling upon the 



