MODES OF ORIGIN COMPARED 51 



of colloidal changes. But, since Rainey's discovery that crystals 

 are produced much more slowly, and undergo very important 

 modifications in shape, when they are formed in viscid solutions, 

 the formation of these bodies has, in both respects, become much 

 more obviously comparable with that of organisms. 



The appearance of these modified crystals may be best watched 

 after mixing solutions of gum and carbonate of potash in the 

 manner carefully described by Rainey. Owing to the viscid 

 properties of gum, a solution of this substance diffuses with 

 difficulty, and hence, when brought into contact with a solution 

 of carbonate of potash, the malate of lime of the gum only decom- 

 poses very slowly. The insoluble carbonate of lime, instead of 

 appearing in its usual crystalline condition, is precipitated in the 

 form of globules resembling calculi. Rainey thus describes what 

 takes place when portions of the two solutions are mixed under 

 the microscope : — " The appearance which is first visible is a faint 

 nebulosity at the line of union of the two solutions, showing that 

 the particles of carbonate of lime, when they first come into 

 existence, are too minute to admit of being distinguished indi- 

 vidually by high microscopic powers. In a few hours exquisitely 

 minute spherules, too small to allow of accurate measurement, can 

 be seen in the nebulous part, a portion of which has disappeared, 

 and is replaced by these spherical particles. Examined at a later 

 period, dumb-bell-like bodies will have made their appearance, and 

 with them elliptical particles of different degrees of excentricity." 

 These modified crystals are, therefore, not produced much more 

 rapidly than the lowest living things appear to be in other solutions 

 during hot weather. The shapes of the products in the two cases, 

 judging from Rainey's figures, are also remarkably similar ; and 

 there is even a deceptive appearance of fission, produced by the 

 juxtaposition of minute specks of carbonate of lime. 



It will be interesting now to turn our attention to fluids contain- 

 ing organic matter in solution such as have been used in experi- 

 ments on so-called 'spontaneous generation,' so as to ascertain 

 what mere microscopical observation can tell us concerning the 

 first appearance of living units therein. 



The writer first made observations of this kind in 1871, using 

 then a strong turnip infusion which had been filtered through 

 several layers of the finest filtering paper. A drop of this fluid was 

 filtered on to a thoroughly cleaned glass slip, over which a large 



