45 



is thrust upon our notice by these experiments, for in both 

 cases we find a material particle of a definite and fixed 

 nature to be the active principle. This active principle may 

 be removed by means, some of which are common to both 

 cases, and we note that just as the lethal germs of disease 

 are carried hither and thither freely in the air, so are the 

 crystalline particles translated ; indeed, so numerous do the 

 particles of a common substance like Sodic Sulphate appear 

 to be in air, that it is hardly possible to expose a super- 

 saturated solution of this substance, to unfiltered air, for at 

 most a few minutes, without crystallisation following. 



We may be pardoned the flight of imagination if we 

 push the resemblance a little farther, to suggest that a 

 supersaturated solution which does not crystallise, though 

 crystals are brought in contact with it, is the case of 

 the healthy man who breathes germ-bearing air with 

 impunity, and the last experiment possesses some interest 

 when thus considered. In a cylinder are two supersaturated 

 solutions, placed in contact one above the other, and con- 

 taining Sodic Acetate and Sodic Hyposulphite respectively. 

 If, now, we drop a washed crystal of Sodic Acetate, it falls 

 harmlessly through the layer of Sodic Hyposulphite being 

 inert to it, but as soon as it reaches the lower layer 

 immediately is active, and crystallisation follows. If, now, 

 a crystal of Sodic Hyposulphite be dropped into the upper 

 and still liquid layer, that, in its turn, falls a victim to the 

 active germ. 



It is interesting to compare the two solutions to a man 

 in good and bad health, subject to the same material germ, 

 the one escaping, the other falling under the baneful 

 influence. 



