20 VETERINAEY HYGIENE 



half a grain of nitric acid per gallon gives a marked 

 colour. 



If nitrous acid be present in the water the above test is 

 unreliable. The nitrous acid can be destroyed by taking 

 about three and a half ounces of the water, mixing with it a 

 few drops of sulphuric acid, and as much pure urea as may 

 be placed on the point of a knife. In the course of an hour 

 the brucine test can be applied. 



Ammonia. — In water this exists in two forms, viz., a.s free 

 or saline ammonia, such as would be found were urea 

 introduced, and secondly, as albuminoid or organic ammonia. 

 Qualitative analysis makes no distinction between these 

 forms, that comes under the head of quantitative, and 

 special methods have to be adopted to obtain them. For 

 ordinary qualitative analysis it is sufficient to know that 

 ammonia is present. 



The test employed is known as Nessler's, which is a 

 compound of mercuric chloride, potash, and potassium 

 iodide ; this reagent possesses the power of detecting the 

 existence of ammonia, even when it is present in very 

 minute quantities. Add to a test-tube one-quarter full of 

 the water under examination, one or two drops of Nessler's 

 solution, shake the tube, if ammonia be present there will 

 be a yellow colour struck at once, or it will come up after 

 standing a few minutes. 



A water may contain free ammonia, and yet give no 

 evidence of it when the test is applied in the above way. 

 The explanation of this is, that in spite of the delicacy of 

 Nessler's test for ammonia, yet (unless the water be con- 

 centrated by distillation) the reaction is not obtained, 

 excepting it is found in large amounts. If on applying 

 Nessler's test to water which has not been distilled a 

 markedly yellow colour forms, there need be no hesitation 

 in rejecting such water for drinking purposes, as the amount 

 of organic matter present must be very large to give such a 

 deep reaction. Should there be only a pale delicate tint of 

 colour, it will render us very suspicious of the water, and 

 exceedingly careful over the analysis. 



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