154 INSECTICIDES, FUNGICIDES, AND WEED KILLERS. 



odour of this product would appear to be unpleasant to insects, and 

 the presence of chloride of lime sufltices to prevent them from laying 

 their eggs. This property has been utilized in arboriculture. For this 

 purpose a mastic is prepared, with 1 part of fat and 2 parts of 

 chloride of lime in powder, and a ring of this preparation applied 

 round the trunk. According to Muhlberg, it suffices to suspend on the 

 tree a small basket filled with chloride of lime. Needless to say the 

 chloride of lime must be renewed as soon as it ceases to emit any more 

 hj^pochlorous acid. 



47. Sulphate of Lime, CaSO,. — Preparation. — Gypsum, or 

 plaster stone, is a natural, crystallized sulphate of lime, CaS0j2H^0. 

 It is found in large quantities in the Trias and Permian. These 

 deposits are the result of the evaporation of ancient selenitic waters and 

 are met with in Morvan, Jura, Saone, Loire, Aveyron, Nievre, Gard,. 

 and also in the environs of Paris, ai-ound Bnghien. Crystallized sul- 

 phate of lime is rapidly and almost completely dehydrated at about 

 120° C, being converted into plaster of Paris, which contains no water 

 but which retains the property of easily taking up its water of crystalliza- 

 tion once more when again moistened. This precious property is lost 

 if the gypsum is heated to 160° C. To obtain sulphate of lime com- 

 mercially the plaster stone is piled up at the quarry mouth in rude 

 arches under a shell made from comlnistible material, or in special 

 furnaces. The blocks must leave between each other interstices of 

 sufficient size to allow the flame of wooden faggots ht in the lower 

 part to pass. A very moderate fire is kept up, which penetrates the 

 mass slowly and dehydrates it. The plaster so obtained is pulverized 

 in mills and preserved out of contact with moisture. 



Properties. — Sulphate of lime is slightly soluble in water ; 1 litre 

 of water at 0° C. dissolves 1-9 grammes ; at 38" C. 2*14 grammes ; at 

 99° C. 1-75 grammes ; it is much more soluble in hydrochloric acid. 



Action of Sulphate of Lime. — Its action is due to its hygro- 

 scopic power and its line division, which asphyxiates the insects by 

 penetrating into the respiratory passages. 



Use of Plaster^ as a Manure. — Known to the Greeks and the 

 Eomans. Manuring with plaster has been in current use, especially 

 since the eighteenth century. Franklin was one of its most zealous 

 advocates. The effect produced on certain plants by sulphate of 

 lime is so violent that scientists are not yet quite agreed as to the 

 explanation. Plaster contains two elements necessary to plants, 

 sulphuric acid and lime ; by supplying these two substances to the 

 soil plants necessarily thrive better than if deprived of these two ele- 

 ments. Although plaster cannot be i-egarded like lime as capable of 

 changing the physical and chemical composition of the soil, it, however, 

 restores to the soil in a soluble form the sulphuric acid and lime re- 

 moved by crops. The effects of plaster may thus make themselves felt 



1 The author does not differentiate between gypsum and plaster but uses the 

 term pldtre throughout. To follow him it will be well to bear in mind that in alL 

 probability when he refers to plaster as a manure he means gypsum, and when he 

 refers to plaster as an insecticide he means plaster of Paris. It is a pity that writers 

 on agricultural chemistry too often lose sight of the fact that superphosphate con- 

 tains naturally about 50 per cent of sulphate of lime in addition to what is added 

 as a " reducer ". — Tu. 



