52 FRUIT TREES AND THEIR ENEMIES 



Its efficacy, and the nature of its action, is a matter of 

 some doubt ; when sufficiently strong — about lo per 

 cent. — brine appears to act on the eggs of insects by 

 osmosis, that is, by drawing the watery contents out of 

 them, and making them collapse ; but it seems doubtful 

 whether such an action can occur with the weaker 

 brines — 2 or 3 per cent. — present in the washes in use. 

 A similar action, however, does occur in the case of 

 the cells of plants, and even a 3 per cent, brine 

 scorches the leaves of apple trees, whilst stronger 

 solutions destroy them altogether. Insecticides con- 

 taining salt should, therefore, only be used while the 

 trees are dormant. 



19. SiilpJuir and snlpJiides are of use as fungicides, 

 especially for surface mildews, and also as insecticides 

 for the destruction of mites, red spiders and aphides. 

 Sulphur by itself is largely used for destroying red 

 spider on hops, when it is dusted on to the hops by 

 means of apparatus specially designed for the pur- 

 pose. It is probable that its action depends on 

 the evolution of a small amount of vapour, for, 

 though it is a solid, it is slightly volatile. When 

 used in hothouses, this volatih'ty may be increased 

 by placing it on the hot-water pipes, the pipes being 

 painted with a mixture of sulphur and water, or 

 sulphur, lime and water. The form of sulphur which 

 should always be used is that known as flowers of 

 sulphur, or else "green" sulphur, which is in a 

 still finer state of division than the flowers. For the 

 treatment of trees it is more convenient to mix the 

 sulphur with water, and spray them, instead of using 

 the dry powder. Sulphur, however, does not mix easily 



