160 LECTURE IX. 



of heat, motion, or otherwise, does not take place : but these 

 facts afford us but little insight. We may admit that during 

 life the atoms in the molecule of protoplasm are in active 

 vibratory movement, whereas in death this movement is 

 arrested, but this does not tell us what is the essential differ- 

 ence between dead proteid and living protoplasm. Though 

 it be granted that in the latter there is this active intra- 



o 



molecular movement, and that in the former it is absent, the 

 question still remains why this is the case. The only kind 

 of answer which has been offered is that there is a chemical 

 difference between the molecule of dead proteid and that of 

 living protoplasm. It is generally held that the molecule of 

 living protoplasm is a very large, complex, and unstable one, 

 and that proteid is only one of many products resulting from 

 its decomposition ; the conversion of dead proteid into living 

 protoplasm would, according to this view, involve the building- 

 up of the complex protoplasmic molecule from the simpler 

 proteid molecule. But whatever the view which may be ac- 

 cepted as to the nature of the protoplasm-molecule, the dif- 

 ference in the properties of living protoplasm and of dead 

 proteid is probably to be ascribed to a difference of molecular 

 structure. Pfliiger conceives it to be this, that in dead proteid 

 the nitrogen is, present in the form of ammonia, whereas in 

 living protoplasm it is in the form of cyanogen. Loew and 

 Bokorny, assuming that in the molecule of proteid a number of 

 aldehyde-groups are present, consider that in living protoplasm 

 the atoms in these groups are in active movement, whereas 

 in dead proteid they are not, in consequence of the altered 

 relation of the- aldehyde-groups to the amide-groups in the 

 molecule. But these are after all mere hypotheses ; we cannot 

 consider that the secret of life has been discovered as yet. 



BIBLIOGRAPHY. 

 I. The Formation of Non-nitrogenous Organic Substance. 



Priestley ; \ 



Ingenhousz ; 



Se'ne'bier ; j see Bibliography of Lecture V. 



de Saussure ; J 



