716 



THE AORICULTilRAL JOURNAL. 



This year I have sent several small 

 parcels of seed to India, and I have at 

 present orders for several hundi'eds of 

 pounds. The present season has been a 

 favourable one, and there is a promise of 

 any quantity of seed in April next. 



I found that 400 lbs. of plant gave from 

 8 to 12 ozs. of dry cake Indigo. 



I am, etc., 



FRIEND ADDIdON. 



Note : — According to the foregoing, 

 25 lbs. of dry Indigo constitutes a fair re- 

 turn per acre. Taisiug the market value 



of Indigo at 3s. per lb., this only gives a 

 return of 75s per acre. This, in my 

 opin on, will not pay. 



T. G. C. 



24th December, 1901. 



It will be seen from the foregoing cor- 

 respondence that Mr. Medley Wood has 

 kindly promised to supplement the in- 

 formation furnished by Mr. Colen- 

 brander, and Col. Friend Addison, when 

 he has received specimens of these In- 

 digo plants. Mr. Medley Wood's article 

 will appear in a future issue of the 

 Jou)-)iaL 



AgHculiural Chemistry for Beginners. 



CHAPTER V. 



By Archibald Pearce, 



PHOSPHORUS AND THE PHOSPHATES. 



THE subject of this chapter is one of 

 considerable complexity, and some 

 parts of it will need rather careful atten- 

 tion ; but on account of the large part 

 played by the pho3.phates in the economy 

 of both plant ;ind animal life, and the 

 many different forms in which they are 

 found, it is often valuable to understand 

 their constitution and peculiarities. Like 

 nitrogen, phosphorus is not of great in- 

 terest in its uncombined state ; it is a 

 yellow waxy solid, so inflammable that it 

 has to be kept under water to prevent it 

 catching fire spontaneously, and is chiefly 

 used m the manufacture of matches. 

 When it burns, which is, as we have 

 learnt, only another way of saying when 

 it combines with oxygen, it forms white 

 clouds of phosphoric oxide, which is one 

 of the acid-forming oxides, and is, there- 

 fore, sometimes called phosphoric anhyd- 

 ride. Making use of our previously 

 gained knowledge of acids, we shall 

 expect to find that this oxide combines 

 with water to form an acid, and this is 

 quite coireet, for it rapidly dissolves in 

 water, forming phosphoric acid. It would 

 also be correct, following what has been 

 said as to the formation of salts, to con- 

 clude that if the hydrogen of the acid is 

 replaced by a metal we shall get a phos- 



phate of that metal. But just here the 

 complexity arises, for phosphoric oxide, 

 in forming the corresponding acid, com- 

 bines with three portions of water, and 

 so it contains its hydrogen in three sepa- 

 rate lots ; and it is found that we can ob- 

 tain three different phosphates, according 

 as one. two, or three of the lots of hydro- 

 gen are replaced by the metal. As we 

 shall be chiefly concerned with the phos- 

 phates of lime, which are more coi-rectly 

 known as the phosphates of calcium, since 

 calcium is the true metal, and lime only 

 its oxide, we will draw up a table show- 

 ing the constitution of these three phos- 

 phates of lime or calcium, and the way 

 they are connected with phosphoric acid, 

 'i he acid may be represented thus : — 



hydrogen 

 hydrogen 

 hydrogen 



phosphorus 

 oxygen 



If one p irtion of the hydrogen be re- 

 placed by calcium, the compound be- 

 comes : — 



calcium phosphorus 



hydrogen oxygen 



hydrogen 



This salt is the mono-calcic phosphate, 

 that is to say, one-lime phosphate, and in 



