308 RADIATION. 



lision, and if these be of a complex clieraical cliaracter tliey instantly clistnrb tlie 

 etliev wliicli sniToancls tlicm and thus lose tlieir heat. Hence the motion pos- 

 sessed in the first instance by the atoms, and which the atoms are incompetent to 

 discharge directly npon the ether, may, by the intervention of more complex 

 molecules, be thus discharged. Suppose, then, a small cpumtity of any vapor to 

 be introduced into an exhausted tube, and air to be subsequently allowed to rush 

 in and fill the tube. By its impact against the sides of the tube the air is heated ; 

 the motion of heat is instantly imparted, b}' collision, to the molecules of the 

 vapor, and they in their turn impart it to the ether, or, in other words, reduce it 

 to the radiant form. By this process, which has been called dynamic radiation, 

 the radiative power of both vapors and gases has been determined, and the reci- 

 procity of their radiation and absorption proved.* In the excellent researches 

 of Leslie, De la Provosta3'e, and Desains, and Mr. ]5alfonr Stewart, the reci- 

 procity of radiation and absorption in the case of solid bodies has been variously 

 illustrated; while the labors, theoretical and experimental, of Kirchoff have given 

 this subject a wonderful expansion, and enriched it by applications of the highest 

 kind. To their resvdts are now to bo added the foregoing, whereby a vast class 

 of bodies hitherto thought inaccessible to experiment are proved to exhibit the 

 duality of radiation and absorption, the influence on both of chemical combina- 

 tion being exhibited in the most decisive and extraordinary way. 



XV. — INFLUENCE OF VIBEATING PEFvIOD AND MOLECULAE FORM. — PHYSICAL 

 ANALYSIS OF THE HUMAN BREATH. 



In the foregoing experiments with gases and vapors we have employed through- 

 out invisible rays. Some of these bodies are so impervious that in lengths of a 

 few feet only they intercept every ray as effectually as a layer of pitch would do. 

 The siibstances, however, which show themselves thus opaque to radiant heat 

 are perfectly transparent to light. Now the rays of light differ from those of 

 invisible heat only in point of period, the former failing to aflect the retina 

 because their periods of recurrence are too slow. Hence, in some wa}^ or other, 

 the transparency of our gases and vapors depends ujion the periods of the waves 

 which impinge upon them. What is the nature of this dependence"? The 

 admiral)le researches of Kirchoff help us to an answer. The atoms and mole- 

 cules of every gas have certain dellnito rates of oscillation, and those waves of 

 eiher are most copiously absorbed whose periods of recurrence synchronize Avitli 

 the periods of the molecules among which they pass. Thus, when we find the 

 invisible rays absorbed and the visible ones transmitted by a layer of gas, we 

 conclude that the oscillating periods of the gaseous molecules coincide witli those 

 of the invisible, and not with those of the visible spectrum. 



It requires some disciidine of the imagination to form a clear picture of this 

 process. Such a picture is, however, possible. Whi^i the waves of ether impinge 

 upon molecules whose periods of vibration coincide with the recurrence of the 

 undulations, the timed strokes of the waves cause the motion of the molecules 

 to accumulate, as a heavy pendulum is set in motion by well-timed puffs of 

 breath. Tiiousands of millions of shocks are received every second from the 

 calorific waves, and it is not difficult to see that every wave, arriving just in time 

 to repeat the action of its predecessor, the molecides must finally be caused to 

 swing through wider spaces than if the arrivals were not so timed. In fact, it 

 is not difficult to see that an assemblage of molecules, operated upon by contend- 

 ing waves, might remain practically quiescent, and this is actually the case when 

 the waves of the visible spectrum pass through a transparent gas or vapor. 

 I.'iiere is here no sensible transference of motion from the ether to the molecules ; 

 in other words, there is no sensible absorption. 



* When heated air imparts its motion to another gas or vapor, the transference of heat is 

 accompanied by a change ot" vibrating period. The dynamic radiation of vapors is rendered 

 possible by the transmutation of vibrations. 



