and the Absorptio7i of the Tithonic Bays. ^SQ 



And further, that when we design to speak less exactly, or 

 merely to indicate in a general way the region to which we 

 refer, we should use the well-known colour designations: thus 

 we might speak of tithonic blue rays, phosphorogenic violet, 

 photic yellow, or thermic red; pointing out at once the im- 

 ponderable principle in action and the spectrum region it oc- 

 cupies. 



II. Laisos of Absorption of the Tithonic Rays. — There is a 

 very great misconception respecting the mechanical force of 

 the tithonic rays. Thus those which come from a candle or 

 other artificial light, are regarded as being exceedingly feeble, 

 and able only to produce an insignificant result. I took a 

 nitric acid battery, of Prof. Grove's construction ; it was suf- 

 ficiently powerful to maintain a platina wire white hot, and 

 decompose muriatic acid with great activity. It was evolving, 

 as Dr. Faraday has made probable, as much electricity as 

 could be found in a thunder-cloud, and yet the rays of a com- 

 mon wax candle could recompose the muriatic acid faster than 

 the battery could decompose it. 



When a beam of light has fallen on any changeable surface, 

 such as a Daguerreotype plate, and is reflected by it, that beam 

 in changing the surface suffers itself a change; for if received 

 on a second similar sensitive surface, it will be found to have 

 lost the quality of giving rise to the decomposition again. 

 Two changes have therefore occurred, — a change in the 

 ponderable body, and a change in the incident beam. 



I passed the rays of a lamp through a mixture of equal 

 volumes of chlorine and hydrogen contained in a plate-glass 

 trough ; the thickness through which the rays penetrated was 

 2'6 inches. The emergent ray was received on a tithono- 

 meter. 



Two distinct phasnomena now appeared : — 1st, the chlorine 

 and hydrogen began to unite; 2nd, the emergent rays had 

 lost much of their power. 



A beam of light, in producing a tithonic effect, becomes 

 proportionally detithonized. Simultaneously the sensitive sur- 

 face becomes tithonized. The beam undergoes a change in 

 producing a change in the ponderable matter. 



The investigation of actinic absorption, therefore, presents 

 itself naturally to us under the two divisions I have already 

 indicated : — 1st, changes taking place in the incident ray; 2nd, 

 changes taking place in the receiving body. 



In this order I shall proceed to consider them, deducing 

 the doctrine of tithonic absorption from one or two special 

 cases, and then extending it to photic absorption. 



