PRESERVATIVES. 187 



shown that when milk containing formaldehyde is distilled, but 

 a small fraction can be obtained in the distillate ; it 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 SchifE'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 

 SchifE'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- 

 chloric acid, it appears that this explanation is not correct ; it 

 is probably due to traces of alkali dissolved from the glass. 



The following tests are a selection from the many which have 

 been devised : — 



(1) To the distillate add one drop of a dilute aqueous solution 

 of phenol, and pour in some strong sulphuric acid down the 

 sides of the tube. In the presence of formaldehyde a bright 

 crimson zone appears at the junction of the two liquids. This 

 test, which is also due to Hehner, is as delicate as the test pre- 

 \iously described, and has the further advantage that it is 

 obtained by formaldehyde solutions of all strengths. If there 

 is more than one part of formaldehyde per 100,000 a white 

 turbidity is obtained in the solution above the sulphuric acid, 

 while in strong solutions a white or pinkish cuidy precipitate is 

 obtained. Many hydroxy-derivatives of benzene, such as salicylic 

 acid, resorcinol, and pyrogallol may be substituted for phenol. 

 Quinol, however, gives not a red colour, but an orange-yellow 

 one. Acetaldehyde gives an orange-yellow colour with phenol 

 and sulphuric acid. 



(2) Mix the distillate with strong sulphuric acid, and sprinkle 

 a little morphine on the surface : a violet colour is produced 

 in the presence of formaldehyde. 



(3) To a decigramme of diphenylamine add 2 c.c. of strong 

 hydrochloric acid, and pour some of the distillate into the warm 

 solution. In the presence of formaldehyde, a white turbidity 

 or precipitate is obtained, on further warming if necessary. The 

 precipitate on prolonged boiling turns green. This test, hke 

 the last, is characteristic of formaldehyde, but is not of such 



