but occurs also in two crystalline forms. It is soluble in water, to a cer- 

 tain extent, the solubility varying- with circumstances. If water at 

 15 C. be shaken for a long- time with the solids, 100 parts of the water 

 will dissolve .28 parts of the crystalline and .92 parts of the powder, 

 while if saturated solutions at ioo u C. be cooled at 15° C, 2.18 parts of 

 the crystalline and 3.33 parts of the powder form remain in solution. 

 Water containing carbon dioxide, however, dissolves much greater quan- 

 tities than does pure water. White arsenic completely and readily dis- 

 solves in solutions of caustic alkalies, such as ammonia, and in solutions 

 of alkaline carbonates, such as washing soda. To both plants and ani- 

 mals it is, along with its compounds, a powerful poison, two or three 

 grains being sufficient to cause death with the human being. 



What is important to know about arsenious oxide in this connection, 

 however, is that with water it forms an acid. For this reason it cannot 

 be used separately as a spray, for it would burn and destroy foliage; it 

 must have its acid or scorching property removed. Now we have in 

 chemistry, compounds which are known as bases, and which combine 

 with acids to form neutral substances called salts. If we treat arsenious 

 acid with a base, we form a salt, termed an arsenite, and this arsenite 

 may be used with water as a spray without fear of doing harm to foli- 

 age. This is what is done, and we have a great many salts of arsenic, 

 such as Paris green, calcium arsenite, sodium arsenite, lead arsenite, etc. 

 In general, all substances containing arsenic are called Arsenic or Ar- 

 senical Compounds. 



But all salts of arsenious acid cannot be used for spraying purposes. 

 Those which are soluble in water, such as sodium arsenite, cannot be 

 employed, and only those which do not dissolve but remain in suspension 

 as solid particles are of use. 



Arsenical Compounds. 



Paris Green. 



This substance is used as an insecticide more largely than any other 

 in the Province of Ontario, due to the fact that it was the first introduced, 

 and, therefore, better known. It is an olive green material consisting of 

 a combination of arsenic, copper, and acetic acid or "vinegar," called 

 by chemists copper aceto-arsenite, along with varying quantities of other 

 substances present as impurities. Theoretically, pure Paris green con- 

 tains 58.65 per cent, arsenious oxide (As 2 5 ), 31.29 per cent, copper 

 oxide (CuO), and 10.06 per cent, acetic acid. Commercially, however, 

 these proportions do not obtain, since there is always a small amount 

 of moisture present in the green together with some sodium sulphate or 

 glauber salt, a compound formed in the process of manufacture and never 

 afterwards completely removed. This latter substance has no insecticidal 

 value, and if present in more than normal quantity only increases the 



