LIVING SUBSTANCE 99 



observed that not only the discs of cross-striated muscle, but in 

 general all fibrous contractile substances, such as those of smooth 

 and cross-striated muscle-cells, the contractile fibres or myoids of 

 the infusorian body, and the cilia and flagella of all ciliated cells, 

 exhibit positive uniaxial double refraction, in such a way that their 

 optical axis coiacides with the direction of the fibres. This fact 

 indicates that the molecular structure of all these fibrous tissues 

 must be different in the direction of the fibres from that in 

 other directions — an inference that is important for the under- 

 standing of the phenomena of contraction in these objects. 

 Engelmann has not been able to find double refraction in the 

 naked contractile protoplasm of Bhizopoda, e.g., Amceba. He ob- 

 served it only in the straight, radiating pseudopodia of Actino- 

 sphcBrmm Eichhornii, a delicate fresh-water rhizopod ; but here it 

 belonged most probably, not to the contractile protoplasm, but 

 to the stiff rays that occur as supporting-organs in the axis of 

 the pseudopodia, and apparently have nothing to do with the 

 contraction. 



D. THE CHEMICAL PROPERTIES OF LIVING SUBSTANCE 



1. The Organic Elements 



Of all the natural sciences, chemistry, in dealing with the atoms, 

 penetrates deepest into the composition of the physical world. It 

 must hence be employed in elucidating the composition of living 

 substance, and thereby completing the preparation for an under- 

 standing of vital phenomena. It is well known that chemistry has 

 arrived at the point at which it recognises the vast variety of 

 substances in the physical world to be composed of the atoms of a 

 small number of relatively simple substances, which thus far it has 

 not succeeded in decomposing. But, although by means of its 

 analytical methods the division of the sixty-eight chemical 

 elements has so far not been accomplished, and their composi- 

 tion out of still simpler substances cannot yet be proved ex- 

 perimentally, no chemist entertains longer any doubt that in 

 reality these elements are not final units. Accordingly, many 

 attempts have been made to arrange them in a genetic relation 

 to one another, and to establish the relationship that is expressed 

 in the analogies of the chemical behaviour of individual elements 

 and their compounds, as a natural relationship arising by the 

 direct derivation of one from another. Especially Mendelejeff, 

 Lothar Meyer, and, most recently, Gustav Wendt and Preyer, 

 arguing chiefly from the relations of the atomic weights of the 

 elements and the similarity of certain elements as regards their 

 own behaviour towards one another and the behaviour of their 

 compounds, have attempted this with success ; the result is shown 



H 2 



