2 BULLETIX 750, U. S. DEPARTMENT OF AGRICULTURE. 
recommended calcium arsenate prepared by the action of soluble 
lime salts on sodium arsenate. In all such cases soluble by-products 
are formed. Since these may influence the action of the spray, 
they should be washed and filtered out if a good grade of com- 
mercial calcium arsenate is desired. 
From the chemical standpoint, the simplest means of producing 
calcium arsenate is the direct union of calcium hydroxid and arsenic 
acid. As the sole by-product of this reaction is water, filtration of 
the material is the only treatment that should be required, thus 
eliminating the washing necessary with most products. 
The experiments here reported were made with the idea of deter- 
mining the proportions and procedure necessary to obtain a product 
having the properties desired for spraying purposes. 4 The arsenic 
acid used was commercial 75 per cent acid, with a density of 1.88, 
corresponding to 61 per cent arsenic oxid (As 2 5 ), or 75 per cent 
arsenic acid (ELjAsOJ. The lime was a good grade of commercial 
lime of the following composition : 
Per cent. 
Loss on ignition 5. 07 
Calcium oxid (CaO) 93. 36 
Magnesium oxid (MgO) 69 
Silicon dioxid (Si0 2 ) 54 
Oxids of iron and aluminum (R 2 3 ) 68 
100. 34 
The most desirable method of uniting lime and arsenic acid is to 
have both ingredients in solution before they are mixed. This neces- 
sitates the use of limewater which contains only about 1 gram of 
calcium oxid per liter, and of a quantity of water so large that it 
probably would be prohibitive in commercial practice. Nevertheless, 
a few experiments were made to discover the nature of the product 
so formed. 
If arsenic acid of any strength is added slowly to limewater con- 
taining phenolphthalein until the latter is just about to decolorize, a 
gelatinous, slow-settling precipitate is formed. After the lapse of 
an hour or so, this crystallizes and settles to about one-eighth of the 
volume of the liquid used. The precipitate filters very readily, 
leaving a cake which on the steam bath dries fairly rapidly to very 
light lumps easily powdered between the fingers. Its extreme light- 
ness is noticeable, and it does not require grinding, simply a disinte- 
1 The desirable features of a good arsenical spray material are as follows: It should not contain an 
excessive quantity of soluble arsenic, which would cause injury to foliage; it should be very finely divided 
so that it may be distributed well over the foliage; it should be light when in the dry form, to permit of 
ready application by dusting; it should suspend well in water, to permit of even distribution when ap- 
plied in the form of a spray; and when sprayed and subsequently dried it should stick well to the foliage. 
