CHAPTER III. 



SIZE OF PARTICLES, MOLECULAR WEIGHT, OSMOTIC PRESSURE, 



CONDUCTIVITY. 



FOR the chemist wishing to discover the constitution of a chemical 

 substance, the determination of the molecular weight is of great im- 

 portance. Much time has thus been spent on determining the 

 molecular weights of the bio-colloids, such as albumin, starch, 

 hemoglobin, etc., and in the following pages we shall show what 

 probability for success attends these efforts. 



A soluble substance, placed in a suitable medium which produces 

 no chemical change, distributes itself uniformly. In the case of 

 crystalloids, it is impossible by either optical or mechanical means 

 to recognize the particles into which it splits up. 1 We shall see that 

 crystalloids are frequently broken up into their molecules. Many 

 colloids are soluble also. If we examine their solutions in the ultra- 

 microscope which permits a hundred thousand fold magnification, 2 

 we can recognize numerous particles. In the case of artificial colloids 

 (gold and silver hydrosols) in which we are certain that all the dis- 

 solved particles are visible, according to R. ZSIGMONDY we are in a pos- 

 ition, as will be shown by the following considerations, to calculate 

 the approximate weight of each individual particle. Let 1 gm. of 

 colloid be dissolved in 1 liter of water, then every 1 cubic millimeter 

 contains 1/1000 milligram of colloid. If by counting under the ultra- 

 microscope, we determine that each cubic millimeter contains 1000 

 particles, we know that each particle weighs one millionth of a mil- 

 ligram. We can easily calculate the diameter of a particle by 

 supplying the specific gravity and assuming that each particle is a 

 sphere. However, as soon as we become uncertain whether all the 

 particles are visible, which is the case with most bio-colloids, the opti- 

 cal method fails us. Under these circumstances, we can determine 

 the size of the particles by ultrafiltration. If we sift grains, we know 

 that those which pass through are smaller, and those which are 



1 [Optical inhomogeneity of sugar solution has been demonstrated. VAN CALCAK 

 and L. DE BRUYN separated sodium sulphate from solution by high speed centrif- 

 ugation. Tr.] 



2 [The ultramicroscope makes visible otherwise invisible particles but does not 

 magnify beyond the power of its component compound microscope. Tr.] 



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