Compafative Physiology. 347 



in producing pictures by pressing substances together, without 

 the aid of light, which is needed in the daguerreotype, have been 

 thought to be effected by latent light. Professor Thomson 

 says, calcined powder of oyster shells, mixed with one fourth 

 part of sulphur, absorbs so much light, that, after being exposed 

 for a few seconds to light, the hours may be seen by it on 

 a watch, when removed to darkness. It loses and recovei's the 

 property of giving out light, by exposure to darkness and light. 

 Heat increases the light of this pyrophorus, he says, and di- 

 minishes its duration. Large pulpy leaves are most productive 

 of action, perhaps from the greater quantity of organic action 

 chiefly ; but j^art of the power may arise from their greater 

 capability of absorbing light. The organs destined to prepare 

 the food for assimilation probably abound most in the leaf, as 

 the greatest changes on the food are produced there, and they 

 are likely to abound more and be more perfect in strong vi- 

 gorous pulpy leaves ; and the action of the sunbeam on these 

 organs, whether by light or heat, or both combined, will probably 

 be more powerful where there is a greater absorbing power. 



On Electricity as a Vital Stimulus, he says : " The mode and 

 degree in which this agent operates on the living system is one 

 of the most obscure but most interesting questions in physiology. 

 If, as there is reason to believe, all the new combinations of 

 elementary substances formed in organised bodies are held to- 

 gether by the same affinities as the inorganic world, namely, 

 by electrical attraction, it is evident that electricity must be 

 regarded as one of the most important of all the vital stimuli, 

 since upon its mode of operation will depend all the earlier stages 

 of the nutritive process. The electricity required will probably 

 be generated within the system itself; since the constant vari- 

 ations in the atmosphere would be attended with too much 

 iincertainty of operation, were living beings dependent on the 

 electricity supplied by it. In all meteorological changes of the 

 atmosphere, alterations in the electric state of the atmosphere are 

 largely concerned ; and the more decided the change, the more 

 evident is the electric disturbance. Many vegetables close and 

 unfold their flowers on the approach and retreat of a storm. 

 In highly electrical states of the atmosphere, young shoots have 

 been observed to elongate with extraordinary rapidity. This 

 effect, however, cannot be imitated by the artificial application 

 of the stimulus, though a gentle current transmitted through 

 the plant seems to increase exhalation, and affect consequently 

 other vital processes ; what is beneficial to some, however, may 

 be injurious to others. In the germination of the seed, the 

 functions are confined to the conversion of starch into sugar, an 

 essentially chemical change, involving the liberation of carbonic 

 and acetic acid. As acids are negative, the seed itself may be 

 regarded in a negatively electric condition ; and, accordingly, it 



