112 Mr. W. Hardy. On the Conditions ivhich 



in which it is suspended."^ An immeasurably minute amount of free 

 alkali causes the proteid particles to move against the stream, while in 

 presence of an equally minute amount of free acid the particles move 

 with the stream. In the one case, therefore, the particles are electro- 

 negative, in the other they are electro-positive. 



Since one can take a hydrosol in which the particles are electro- 

 negative and, by the addition of free acid, decrease their negativity, and 

 ultimately make them electro-positive, it is clear that there exists some 

 point at which the particles and the fluid in which they are immersed 

 are iso-electric. 



This iso-electric point is found to be one of great importance. As it 

 is neared, the stability of the hydrosol diminishes until, at the iso- 

 electric point, it vanishes, and coagulation or precipitation occurs, the 

 one or the other according to whether the concentration of the proteid 

 is high or low, and whether the iso-electric point is reached slowly or 

 quickly, and without or with mechanical agitation. 



This conclusion can be verified experimentally in many ways. If a 

 coagulum or precipitate of the proteid particles made either by the 

 addition of a neutral salt, or by the addition of acid or alkalis, be 

 thoroughly washed, made into a fine mud in an agate mortar, and 

 suspended in water in a U-tube, it rapidly subsides. The establishment 

 of an electric field having a potential gradient of 100 volts in 10 cm. 

 has no influence on the level of water or precipitate in forty-eight 

 hours. If, now, the smallest possible amount of caustic soda or acetic 

 acid be added, the proteid will commence to move, so that in twenty 

 hours the precipitate will rise in one or other limb until it nearly 

 touches the platinum electrode. 



Speaking generally, the hydrosol of ferric hydrate is stable only in 

 the absence of free acids or alkalis or neutral salts. The hydrosol of 

 heat-modified proteid is stable only in presence of free acid or alkali. 

 The hydrosol of gum mastic is readily precipitated by acids, but is 

 stable in presence of any concentration of monovalent alkalis. The 

 general conditions of stability of these various hydrosols, therefore, are 

 very diff'erent, yet they agree in manifesting the same important rela- 

 tion between the isoelectric point and the point of precipitation as is 

 shown by the hydrosol of proteid. 



In the hydrosol of ferric hydrate the particles are markedly electro- 

 positive. A dilute hydrosol is coagulated by citric acid when the 

 concentration of the latter reaches 1 gramme-molecule in 4,000,000 c.c. 

 No matter how small the concentration of the ferric hydrate, the 

 hydrosol becomes cloudy and settles. The rate of settling is, however, 

 slow, being about 1 cm. an hour. In an electric field, having the form 

 of a U-tube, the particles always settle slightly faster from the negative 



* "The Coaerulation of Proteid by Electricity," W. B. Hardy, 'Journal of 

 Physiology,' vol. 24, 1899, p. 288. 



