26 Mr. Hunt on Actino-Chemistry. 



mena. It has indeed been suggested they may be distinct 

 principles^ and a considerable amount of experimental evidence 

 has been brought forward in support of this position. With- 

 out entering into the argument in this place, I desire simply 

 to impress strongly the necessity of considering the three effects 

 of luminous agency and colour, of calorific action, and of 

 what has been called photographic influence, as phaenomena 

 depending upon three things (whether these be independent 

 principles, or only modifications of one great immaterial ele- 

 ment, not being here considered), to which it is necessary to 

 give distinctive appellations. Light and heat are terms about 

 which, in their familiar acceptation, there can be no mistake; 

 but it is to be regretted that some confused terms have had 

 the sanction of some eminent experimentalists, leading, as it 

 appears to me, to a sad complexity of ideas. An " invisible 

 chemical coloration" is the hypothesis of one philosopher and 

 " invisible light " the epithet of another, introduced in me- 

 moirs of great interest, in which the changes produced by the 

 dark chemical rays, as they have been called, are sought to 

 be explained. In support of this view it has been suggested 

 that rays of light may exist which do not produce any excite- 

 ment of the optic nerves of man, and it has been assumed that 

 these rays may still be sufficiently powerful to produce vision 

 in the night-roving animals ; howbeit, of this there is not the 

 slightest proof, and all the phaenomena of vision in the cat, 

 owl, and the like, in comparative, not absolute darkness, may 

 be physiologically explained. However, in this paper Light 

 will be used to distinguish those radiations which produce 

 vision and colour. Heat, those affecting any thermic phte- 

 nomena; whilst those radiations on which certain chemical 

 changes are supposed to depend will be distinguished by the 

 epithet of Actinic. 



At the Meeting of the British Association at York, it was 

 proposed by Sir John Herschel that all those phaenomena 

 which exhibit change of condition under the influence of the 

 solar rays should be distinguished, as forming a peculiar pro- 

 vince of chemistry, by the term Actino-Chemistry, and this 

 was generally approved by the Chemical Section. Actinism 

 it is proposed shall in future be used to express that principle 

 or modification upon which these phaenomena depend. Ac- 

 TiNiciTY will distinguish this power in action, and Actinized 

 I shall use to signify any substance which has been exposed to 

 Actinic influence. In the progress of the examination of 

 these physico-chemical phaenomena other epithets must of ne- 

 cessity be formed ; for example, it will be convenient to speak 

 of media which obstruct the passage of actinism, like the qua- 



