COLLOIDS 239 



cation as a hard ruby solid, presenting all the transitions between 

 the gold fluid and the ruby glass." (Bakerian Lecture, 1857.) 



Some of these metallic sols are now stated to have a germicidal 

 action which may result in valuable medical results. The late 

 Mr. Henry Crookes has introduced certain preparations of this 

 kind containing silver and mercury which he called " collosols." 

 They are stated to be fatal within a few minutes to all the 

 pathogenic organisms examined, at the same time that they have 

 no injurious effects on animal tissues. These liquids contain the 

 metallic particles in a state of suspension and so small as to 

 exhibit the Brownian movement actively. How they attack 

 bacteria is not certain, but it appears probable that the particles 

 in any one preparation being all in the same electrical state, 

 positive or negative, do not touch one another and do not lose 

 their charge. When a neutral foreign body such as a microbe 

 is introduced into the liquid it probably receives the charges of 

 many thousands or millions of particles of metal, and this may 

 account for its death. Favourable notices concerning the germi- 

 cidal power of these collosols have been published both in the 

 Lancet (12th December, 1914) and the British Medical Journal 

 (16th January, 1915). 



We may now turn again to the consideration of the question 

 of the sizes of particles which are recognisable by the ultramicro- 

 scope. So far as molecules of gases are concerned it has already 

 been pointed out that the probability of their being ever seen is 

 remote. But since something is now known of the molecular 

 size of the molecules of some albuminoid matters, all of which 

 are undoubtedly very large, it seems possible that in such cases 

 the molecule may be seen. Experiments on the diffusion of 

 various colloids by llerzog and Kasarnowski in 1908 led to the 

 following figures, which are of course only approximations and 

 are to be compared with the molecular weight of hydrogen as 2. 



Substance. Molecular weight. 



Egg albumin 17,000 



Pepsin 13,000 



Invertin 54,000 



Emulsin 45,000 



Calculating the molecular diameter of such bodies the largest 

 molecules reach the size represented by 6 JULJUL, which brings them 

 within the range of the ultra-microscope, It is obvious, how- 



