362 CHEMISTRY. 



on the part so prepared, making the letters legible to every 

 passer-by, brought the new manure at once into popular 

 favor. There has been much dispute as to the manner in 

 which gypsum acts as a fertilizer. One thing is quite set- 

 tled about it it answers a good purpose infixing the am- 

 monia in the soil. This is effected by a double decompo- 

 sition between the sulphate of lime and the carbonate of 

 ammonium, the result being carbonate of lime and sulphate 

 of ammonium. In this connection we will mention that sul- 

 phuric acid is often used for fixing ammonia in manures, 

 forming with it a sulphate, which is not volatile like the 

 carbonate. 



506. Vegetable Refuse. In every garden and on every 

 farm all vegetable matter which is useless should, so far as 

 it can be done, be made to add to the stock of humus by 

 its decay. It is convenient to have in a garden a pit into 

 which all weeds, small trimmings, etc., can be thrown, 

 where, covered up, they may be left to decay, forming rich 

 humus. The decay may be hastened by the occasional ad- 

 dition of some lime. On most farms there is a large quan- 

 tity of vegetable matter left to decay on the surface of the 

 ground, and thus waste by volatilization a part of its fer- 

 tilizing material. This refuse might be of great value if 

 gathered up and mingled in a compost heap with other 

 materials. 



507. Sewer- Water. This always contains a great variety 

 of fertilizing substances, and therefore is one of our most 

 valuable manures. Yet it is very generally wasted. Vast 

 quantities of it in our towns and cities run off into the wa- 

 ter, where it is not only lost, but sometimes does much 

 harm. The water of the River Thames is becoming more 

 impure every year from this cause. It is calculated that 

 the London sewers pour into it fertilizing materials of the 

 annual value of over half a million pounds sterling. Great 



