LIFE AND DEATH. 273 



chemical elements must be given in certain definite 

 proportions and forms: for instance it is of no use 

 to offer the carbon and sulphur in such a form 

 as carbon disulphide, or the nitrogen and hydrogen 

 in that of hydrocyanic acid, but the carbon must 

 be given to the protoplasm in the form of a car- 

 bohydrate or in some similar form, the nitrogen 

 as an ammonium salt, nitrate or proteid, the 

 sulphur as a sulphate, and so forth, and thus 

 water, air, carbohydrates, and the nitrates, sul- 

 phates, and phosphates of potassium, calcium, 

 and magnesium become the chief natural sources 

 of the essential ingredients. Again, we have 

 learnt that while there are different forms of 

 protoplasm in the cell, and that these react on 

 each other, and go through cycles of arrangement 

 and rearrangements, the intimate structure must 

 be of that kind termed molecular beyond the 

 region of vision, just as is the microscopic structure 

 of a crystal ; but, while like the latter affording 

 evidence of order and sequence when properly 

 examined, the structural arrangements and changes 

 must be infinitely more complex. 



All these, and numerous other results of 

 enquiry, have led to the conclusions that we must 

 regard living protoplasm as a complex made up 

 of very large molecular units, each containing 

 atom-groupings of the elements named ; and, 

 partly on account of the large number of atoms 

 they contain, and partly due to the vibrations of 

 absorbed heat, these units must be extremely 

 labile. Moreover, they are linked up into an 



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