440 Dr. E. J. Mills on Colorimetry. 



by its means — this process being repeated until there is only 

 a difference of a division or two between the two liquids. 



The second approximation will in general be found suffici- 

 ently exact. All dilutions should as far as possible have the 

 same age. With regard to the standard tint selected, the ope- 

 rator has in this colorimeter the means of varying his standard 

 to any extent by shifting one opal disk ; he can thus work at 

 the particular depth of tint which he rinds most suitable to his 

 own eye. Steady accuracy in any particular measurement 

 can generally be obtained by at most a few days' practice. 



Turbidities. — In connexion with the Portable Colorimeter, 

 I pointed out* that a black or coloured disk, lowered through 

 a turbid liquid, eventually vanishes, and that the depth at 

 which disappearance takes place is a measure of the amount 

 of turbidity present. In this way, for example, it is easy to 

 estimate the amount of water added to milk. It is obvious, 

 however, that this method admits of quantitative extension to 

 all sorts of precipitates, provided we can find a suitable medium 

 to ensure their . suspension as a turbidity, and not in the ag- 

 gregated state, during a s itable time. 



The suspensory liquid I now employ consists of 100 grms. 

 of gelatine, 100 grms. at most of glacial acetate (" acetic 

 acid"), and 1 grm. of salicylate ("salicylic acid") dissolved 

 in a litre of distilled water : this is clarified with a little 

 white of egg, and filtered hot. It remains permanently liquid 

 in the cold, and does not putrefy. It may, if desired, be 

 charged with any special reagent (baric chloride for instance): 

 a volume of the mixture can then be added to a volume of a 

 very weak standard sulphate, and also to a volume of sulphate 

 of unknown strength ; by depressing the black buttons, the 

 colorimeters determine the relation between the two. The 

 reacting bodies should in such cases be the same; thus, hydric 

 sulphate should not be compared against potassic sulphate. 

 The key to success in colorimetry is, in fact, equality of con- 

 dition. 



If the precipitant should be alkaline, or an alkaline carbo- 

 nate, the gelatine solution should first be neutralized and then 

 mixed with more alkali or carbonate. Such solutions as 

 aqueous magnesic chloride and zinc sulphate can then be 

 added, the whole instantly well shaken, and the result com- 

 pared with a standard effect in the other tube. 



Lime can be determined by adding ammonia and amnionic 

 oxalate to the suspensory liquid, and then a weak solution of 

 calcic salt. 



There is probably no substance incapable of suspension for 

 * Loc. cit. p. 312. 



