DETECTION OP ADDED WATER IN MILK. 161 



worked with a difference of 10° C. between the atmospheric tempera- 

 ture and that of the. solution knows how very great are the difficulties 

 of getting- correct readings. I have found it fairly easy to keep the 

 cooling bath constant to 0'1° C, but difficult to keep the solution and 

 the prism of the refractometer exactly constant in temperature. 

 Apart from these working difficulties we have the fact that the varia- 

 tions in the refractometer reading from different samples of genuine 

 milk are rather great. According to Leach, they vary from 39 to 

 44'5; according to McCrae, using the same method, from 39 to 46"5 ; 

 and, according to Ackerman, from 38 '5 to 40' 5. Then, according to 

 Leach's work, 1 deg. on the refractometer is equivalent to about 4 

 per cent, of added water; and, according to Ackermann, to about 6 

 per cent, of added water. This means that, to a good milk of high 

 refractometer reading, about 20 per cent, of water could be added 

 without bringing the milk under the minimum of 39 deg. 



This is, I think, the most important objection to this process. 



Tlie freezing point of milk was, as long ago as 1895, affirmed by 

 Winter to be practically constcxnt ; as a result of thousands of tests 

 it has now been found that the mixed milk of herds of cows never 

 varies beyond — 0"5o° and - 0'56° C, the mean being - 0*555° C. 



Winter's process of determining the freezing point of the milk is 

 easy and accurate. The freezing point of 12 samples of milk can 

 easily be determined in an hour, and in testing the process the follow- 

 ing results were obtained : — Six i-esults obtained by three operators, 

 each doing two, varied from — 0'555" to — 0"557°, or a maximum 

 variation of "002° C. Similarly three results on one sample gave 

 '000° C. variation, and three other tests in triplicate gave '001°, 

 '001°, and '001° C. maximum variation. 



The working error in the process is, therefore, very small, being 

 equivalent to less than 0'4 per cent, of added water, while the maxi- 

 mum variation in the milk of dift'erent herds is equivalent to less than 

 4 per cent, of added water. 



That such an easy and delicate process for the determination of 

 adde<l water should have been ignored for so long by English-speaking 

 chemists is a standing monument to their conservative instincts. 



Although much work has been done in this direction on the 

 Continent, I can only find one reference to it in England, that by 

 Atkins in the "Chemical News" for 1908, Vol. 97, page 141, and in 

 that short article no mention whatever is made of the work already 

 dene in this direction. 



Towards the close of 1907 I made a few experiments on the 

 freezing point of milk, testing milks of known purity and milks with 

 known added water, and the results w^ere so satisfactory that I con- 

 tinued the work as occasion permitted. Finally, about seven months 

 ago, I determined to utilise the method for checking all those milks 

 which were of doubtfulquality. I append the results in the following 

 table. As none of the samples were curdled, the acidity was rarely 

 determined. It is evident that in the case of at least eight of the 

 samples sufficient acidity had developed to lower the freezing point, 

 as the added w^ater calculated from that figure is less than that 



