307 
are nitre, or the nitrous acid combined with potassa. 
Sir. Kenelm Digby states, that he made barley grow 
very luxuriantly by watering it with a very weak 
solution of nitre ; but he is too speculative a writer 
to awaken confidence in his results. This substance 
consists of one proportion of azote, six of oxygene, 
and one of potassium ; and it is not unlikely that 
it may furnish azote to form albumen or gluten in 
those plants that contain them ; but the nitrous 
salts are too valuable for other purposes to be used 
as manures. 
Dr. Home states, that sulphate of potassa, which, 
as I just now mentioned, is found in the ashes of 
some peats, is a useful manure. But Mr. Naismith # 
questions his results ; and quotes experiments 
hostile to his opinion, and, as he conceives, un- 
favourable to the efficacy of any species of saline 
manure. 
Much of the discordance of the evidence relating 
to the efficacy of saline substances depends upon 
the circumstance of their having been used in dif- 
ferent proportions, and in general in quantities 
much too large. 
I made a number of experiments in May and 
June, 1807, on the effects of different saline sub- 
stances on barley and on grass growing in the same 
garden, the soil of which was a light sand, of which 
100 parts were composed of 60 parts of siliceous 
sand, and 24 parts finely divided matter, consisting 
of 7 parts carbonate of lime, 12 parts alumina 
and silica, les& than one part saline matter, prin* 
* Elements of* Agriculture, p. 78. 
x 2 
