198 ELEMENTAKY AGRICULTURAL CHEMISTRY 



The chief *' preservatives " vised are : 



1. Boric acid, HBO3, or borax, NagBp^.lOHjO. 



2. Salicylic acid, CeH,(OH).COOH. 



3. Formaldehyde, H.CHO. 



4. Sodium carbonate, NagCO^. 



5. Glycerine, C3H,(OH)3. 



6. Benzoic acid, C6H,.C00H. 



7. Beta-naphthol, C^JIfiJI. 



No. 4, sodium carbonate, is not a true preservative, since it 

 does not prohibit the activity of the lactic acid organisms, but, 

 indeed, rather favours their action. However, by neutralising 

 the lactic acid as fast as it forms it postpones the curdling of 

 the milk. Its presence is readily detected by ashing some of 

 the milk and adding hydrochloric acid, when an effervescence 

 will /show the presence of carbonates. 



The favourite preservatives are formaldehyde and boric 

 acid. 



Formaldehyde is a gas, very soluble in water. A solution 

 containing 40 per cent, of real formaldehyde is commercially 

 known as ** formalin," and is the origin or source of many 

 commercial milk preservatives. These substances usually 

 contain from 1 to 6 per cent, of real formaldehyde in water, 

 and they are generally added in the proportion of about 

 1 ounce to 10 gallons of milk. The milk thus receives 1 part 

 of the real preservative to from 20,000 to 50,000 of milk. 

 Even in these small proportions the preservative power is 

 remarkable, but this increases greatly as the quantity added is 

 increased; thus 1 part of formaldehyde in 50,000 of milk 

 extended the time required for curdling from 36 hours to 66 

 hours, the mi k being kept at 20° C. 1 part in 20,000 

 extended the time required for curdling to 96 hours, while 1 in 

 10,000 required 51 days, 1 in 5000 lO^ days, and 1 in 2500 

 kept the milk from curdling for 55 days. It is very doubtful 

 whether formaldehyde is as efficacious in destroying pathogenic 

 germs as it is in preventing the lactic fermentation. 



