36 FIELD, FOREST AND FARM 



small dose, may become harmless and even enter into 

 the composition of our food when they are combined 

 with other substances. Thus it is with phosphorus. 

 What, then, is united with phosphorus in the form in 

 which it ceases to be poisonous and enters into the 

 composition of meat and flour ? That is what we will 

 now consider. 



"When phosphorus is burned it produces a thick 

 white smoke, of which you can get some idea by 

 striking a number of matches all at once. This white 

 smoke with the slightest trace of humidity is redu- 

 cible to an extraordinarily acid liquid called phos- 

 phoric acid. Since this compound results from the 

 combustion of phosphorus, just as carbonic acid is 

 the result of the combustion of carbon, it must and 

 in fact does contain the air without which no com- 

 bustion can take place. Phosphoric acid is no longer 

 inflammable, however much it may be heated ; being 

 itself the product of combustion, it cannot burn again. 

 But if there is no danger of its catching fire, phos- 

 phoric acid is nevertheless dangerous on account of 

 its intense acidity, which makes it violently corrosive 

 in its action on flesh. If mixed with lime, however, 

 this formidable compound loses its injurious proper- 

 ties and is changed into a white substance without 

 the least taste or the slightest poisonous effect. 

 This substance is called phosphate of lime. Burnt 

 phosphorus and lime, thus united, furnish the greater 

 part of the mineral matter found in bones. Put a 

 bone into the fire : the grease and juices that perme- 

 ate its substance will be burnt up and the bone will 



