206 DEFECTS OF FORMER EVIDENCE. 



admitted. It was only after many years of discussion, and multitudes of experiments, 

 that the doctrine of the unity of air was destroyed, and the theory of the intrinsic dif- 

 ferences of gaseous bodies received. This was unquestionably the most important event 

 that ever happened to chemistry. 



939. The imponderable principles are the true LIVING FORCES of chemistry. The 

 circumstance that they do not exhibit the property of weight is only an incidental af- 

 fair, and ought never to have been regarded as their leading characteristic : they are 

 the regulating forces by which ponderable matter is arranged and grouped. If, then, 

 so great a change occurred in chemistry, on more exact views being obtained of its 

 pneumatic department, what may not be expected from the discussions which are ari- 

 sing on the nature of its great controlling forces ! 



940. There is another point of view from which these investigations assume a 

 deep interest. I have shown in Chapter XV. that, by resorting to prismatic analysis 

 in physiological researches, very remarkable truths appear. The function of diges- 

 tion, which is carried on during sunshine by the leaves of plants, is under the control 

 of the yellow ray. It is this which causes the decomposition of carbonic acid, furnishes 

 solid food, and gives the green colour. In animals similar results are produced by the 

 agency of a nervous system ; and not only so, but all the various operations connected 

 with life are conducted in the same way. There is one class of nerves which gives 

 action to the respiratory apparatus, and another which controls digestion. There is 

 one class which presides over motion, another which is the recipient of sensation, a 

 third which originates all the processes of thought and intellectuality. In the vegeta- 

 ble world the same idea is preserved, developed, perhaps, in a less elaborate way, but 

 under the guidance of a principle equally ethereal and refined. The beams of the sun 

 are the true NERVOUS PRINCIPLE of plants. To the yellow ray is assigned their nutri- 

 tive processes, to the blue, their movements. We can, therefore, easily understand how 

 it is that botanists, who have sought in the interior of plants for indications of a ner- 

 vous agent, have never found them. That agent is external. 



941. By the experiments that have been made for determining the nature of the 

 chemical radiations, the question has been brought down to very narrow limits. There 

 is no author who regards them as connected in any way with radiant heat, nor any, 

 except the wildest speculator, who traces them to electricity. The difficulty is, to offer 

 clear and undoubted proof that they are distinct from light. HERSCHEL has directly 

 admitted this distinction, and brought forward several experiments (Phil. Trans., 1840, 

 p. 38, &c.) in support of this view. I have given some evidence of the kind, both re- 

 cently and also several years ago (1837). All these experiments depend on a com- 

 parison of tithonographic stains produced by solar spectra that have undergone the ac- 

 tion of absorptive media, and the effect of those spectra on the organ of vision ; a com- 

 parison, in short, of different visible spectra and their tithonographic impressions. 



942. From this comparison, we endeavour to prove that invisible rays may be iso- 

 lated in any part of the spectrum, and, if invisible, we argue that they are not light. 



943. It cannot be concealed, however, that there is a certain degree of imperfection 

 in this species of evidence : an accurate conclusion as to the presence or absence, or 



