APPLICATION OF ANALYSIS 61 



is developed, and the liquid squirts out ; milk containing 

 i per cent, of urine yields about two -fifths the volume 

 of gas, while 5 per cent, causes an evolution of gas 

 equal in volume to the milk taken. Lobeck's catalase 

 tubes (p. 39) may be used. 



The Cause of Dirty Milk. Dirty milk almost in- 

 variably deposits a sediment on standing or centri- 

 fuging ; the milk is carefully decanted, the sediment 

 washed with water, and allowed to settle again and 

 examined under the microscope (Fig. 30). Sharply 

 defined vegetable ceils (Fig. 31), indicate that the finer 

 particles of the food given to the cattle probably at 

 milking time have fallen in ; less well defined vegetable 

 cells (Fig. 32), stained yellowish are probably derived 

 from faecal matter ; small hairs and various fibres 

 (cotton, wool, &c.) show the presence of household dust ; 

 transparent irregular particles which do not polarise are 

 quartz, and are due to road dust ; this latter also gives 

 a strong reaction for iron on treating the sediment with 

 hydrochloric acid, diluting, and testing with potassium 

 ferrocyanide which gives a blue colour with iron salts. 



Control of Milk Prescriptions. To control the 

 preparation of milk mixtures made up for infant feeding 

 from prescriptions, it is necessary to estimate rapidly 

 fat, proteins, milk sugar, and added sugar. The problem 

 is often simplified by all the materials used being 

 available for analysis, and by determining fat (by the 

 Gerber method), specific gravity, and aldehyde figure, 

 and calculating the solids not fat, in the samples and 

 in the milk from which the samples were made, the 

 four determinations required can be obtained with 

 sufficient accuracy for control purposes. 



By estimating the ratio of solids not fat to aldehyde 

 figure in the milk used, and multiplying the aldehyde 

 figure of the mixture by this ratio, the amount of solids 

 not fat derived from the milk is obtained, and the 

 difference between the actual amount and this gives 

 the added sugar. With an unknown milk mixture 

 the ratio 0.45 may be assumed. The method does not, 



