1878.] the Influence of Light upon Protoplasm. 209 



water dissolves that gas, we shall at once see that the Torula, deriving 

 its respiratory oxygen from the sugar of the solution, is all this time 

 living comparatively in absence of free oxygen, and we understand 

 how the relations of protoplasm to oxygen, by enabling it in some 

 forms to be largely independent of the uncombined gas, may prove a 

 source of protection against the oxidising action of light. 



In some cases, indeed, the affinity of organisms for oxygen would 

 appear to be so great that, when presented to them in its gaseous and 

 uncombined state, it acts, not as a source of vital energy, but as a 

 poison, and we think that protoplasm will be found to possess varying 

 degrees of tolerance of excess or deficiency of this element. To some 

 forms of life, if Pasteur be right, oxygen is injurious even when 

 diluted as in ordinary air, to others it is hurtful only when oxidation 

 is quickened by some adjuvant force, as, for example, by light. 

 Finally, since light here acts as an oxidiser, it is conceivable that 

 there may exist sluggish forms of protoplasm, whose oxidising pro- 

 cesses, and, therefore, general growth and development, may be 

 favourably augmented by a modified degree of light. We are not of 

 our present knowledge, however, able to point to such.* 



In connexion with the subject of this paper, it is an interesting 

 speculation whether any one of the constituent elements of organic 

 bodies is specially subject to oxidation under light. We seem to have 

 obtained some glimpse of a possible answer to this question by a 

 few experiments upon the oxalates. If the constitutional formula 



C — — — H 



of oxalic acid be rightly represented thus, ||| , a mode of 



C — — — H 



approach to the problem seems to be opened, for should we find on 

 substituting some other element for the hydrogen that decomposition 

 is no longer produced by light, the conclusion would seem inevitable 

 that the destruction of the molecule of oxalic acid was effected through 

 the oxidation of the hydrogen. 



On July 26th a solution of neutral oxalate of potash of decinormal 

 strength was divided between a number of test-tubes, some of which 

 we incased while some were insolated in the usual way. At the same 

 time a decinormal solution of oxalic acid was similarly treated. 



August 26th. The insolated oxalic acid solution is completely decom- 

 posed, but the incased oxalic acid solution is unaffected. The solutions 

 of oxalate of potash, both incased and insolated, remain quite un- 

 changed, and are still neutral to test-paper. 



* From what we have said, it would follow that the organisms most injuriously 

 affected by light would be found to be those whose protoplasm is "unprotected," 

 having high affinities for oxygen, but yet for the most part requiring it uncombined, 

 and at the same time being so minutely particulate as to offer in .point of surface the 

 greatest facility for access both of light and of oxygen, all of which conditions are 

 exemplified by the ordinary forms of Bacteria. 



