FORMALDEHYDE. 209 



slight greenish tinge at the junction of the two liquids, and on 

 standing a brownish colour is developed, not at the junction of 

 the two liquids, but lower down in the acid. 



Leonard and Smith's test for formaldehyde consists in heating 

 a little milk with 3 to 5 times its volume of concentrated hydro- 

 chloric acid ; a fine violet colour is produced in the presence of 

 formaldehyde (0-0001 per cent, to 0-1 per cent.). The presence 

 of a trace of ferric chloride in the hydrochloric acid is 

 essential. 



These tests are not absolutely characteristic of formaldehyde, 

 and are not given in the presence of large amounts of this body. 

 It is a reaction of the tryptophane of the casein with formalde- 

 hyde, and certain other aldehydes e.g., vanillin give similar 

 colours. Leonard has pointed out that pure acids give no re- 

 action, but the presence of an oxidising agent is necessary ; 

 he found that a trace of ferric chloride gave the best results ; 

 it is better to use commercial acid than a purer form, as the 

 necessary oxidising agent is present. 



As a confirmatory test, some of the milk may be curdled by 

 dilute sulphuric acid and a little Schiff's reagent a solution of 

 rosaniline bleached by sulphurous acid added to the nitrate in 

 a test tube, which is corked and allowed to stand. In the pre- 

 sence of an aldehyde a violet-pink colour is produced after a 

 short time. Excess of sulphurous acid must be avoided in 

 preparing the reagent, or the test may fail with small amounts. 



There are many confirmatory tests, which are best applied to 

 the clear solution obtained by distilling the filtrate obtained by 

 curdling the milk with sulphuric acid. Smith and Leonard have 

 shown that when milk containing formaldehyde is distilled, but 

 a small fraction can be obtained in the distillate ; if the milk be 

 made alkaline, still less is obtained ; but a very much larger 

 proportion is obtained by distilling from an acid solution. This 

 is due to the fact that formaldehyde condenses with the proteins 

 of the milk ; the more perfectly these are in a state of solution, 

 the faster is the rate of combination. Combination is more 

 rapid at high temperatures, but takes place at ordinary tem- 

 peratures, and the total quantity added is never obtained ; after 

 a lapse of some time several days the formaldehyde disappears, 

 and can no longer be detected. 



If Schiff's test is applied to the distillate, it must be rendered 

 faintly acid beforehand with hydrochloric acid ; Hehner has 

 shown that the distillate of milk gives a faint pink colour with 

 Schiff's reagent after some time, but this disappears on the addi- 

 tion of a drop or two of sulphurous acid, while the colour due to 

 the presence of formaldehyde does not. He ascribes this to 

 oxidation, but as it is equally well prevented by a little hydro- 



14 



