546 SOURING AND PRESERVING OF MILK. 



sugar, and of the latter into lactic acid, may be brought about, and seem 

 almost to understand liow it is done, since it can be effected by a simple 

 transposition of their elements. But the after-thought occurs — by what 

 kind of power is this change effected ? The materials are certainly pre- 

 sent, but how are they made to shift their relative positions, and move 

 into their new places ? We have concpiered one intellectual difficulty 

 only to encounter another apparently still harder to overcome. 



It was said first, I believe by Priesl*ey, [Experiments and Obser- 

 vations, ii., p. ix., edition 1781,] " that the greater the circle of light, 

 the greater is the boundary of darkness by which it is confined." Thus 

 they who know the most are the most strongly impressed with the sense 

 of their own want of knowledge. What a fine result this is of large 

 acquirements ! And how touchingly it was expressed by Sir Isaac New- 

 ton, when he likened his great discoveries to the gathering of a few peb- 

 bles along the sea-shore — the vast ocean of natural knowledge lying still 

 unexplored before him I 



§ 7. Of the souring and preserving of milk. 



The natural souring of milk requires now little explanation. It arises 

 from the gradual conversion of the sugar into the acid of milk by the 

 action of the casein. There are, however, one or two circumstances con- 

 nected with it to which it may be proper to advert. 



1°. If milk be kept at a low temperature, it may be preserved for se- 

 veral days without becoming sensibly sour. This is effected in Switzer- 

 land by immersing the milk vessels in a shallow trough of cool water, 

 which, by means of a running stream, can at any time be renewed. In 

 such circumstances the action of the cheesy matter in converting the 

 sugar into lactic acid is very slow. 



2°. But if the milk be kept at the temperature of 65° or 70° F. it be- 

 comes sour with great rapidity, and if afterwards raised to the boiling 

 point curdles immediately. An easy way of preserving milk or cream 

 sweet for a longer time, or of removing the sourness when it has already 

 come on, is to add to it a small quantity of the common soda, pearl ash, 

 or magnesia of the shops. Enough is added, when a little of the milk 

 poured into boiling water no longer throws up any curd. As the small 

 quantity of soda or magnesia thus added is not unwholesome, cream 

 may in this way be kept sweet for a considerable time, or may have its 

 sweetness restored when it has already become sour. 



3°. I have already observed to you that animal membrane, the curd of 

 milk, or any of tlv/se substances which possess tlie power of changing sugar 

 into lactic acid, loose that power if the solution in which they are present 

 be raised to the boiling temperature. Hence if milk be introduced into 

 bottles, be then well corked, put into a pan with cold water, and gradually 

 raised to the boiling point, and after being allowed to cool be taken out 

 and set away in a cool place, the milk may be preserved perfectly 

 sweet for upwards of half a-year. 



I mentioned also that if the solution containing the sugar and cheesy 

 matter be again exposed to the air after boiling, it will gradually resume 

 the property of transforming the sugar into lactic acid. Hence, if milk 

 be boiled, it is preserved sweet for a longer period of time, but the 

 casein gradually resumes its transforming property, and at the end of a 



