[ 207 ] 
necessary for those purposes, and as gypsum^ con¬ 
sists of sulphuric acid and lime, it answers the 
most beneficial purpose on gravelly soils, which 
therefore seem to contain too little acid. That it 
is the acid that acts principally in the use of gyp“ 
sum, seems still more probable from the known 
property of the sulphurous acids, to dislodge most 
if not all the other known acids from their earthy 
and metalic bases, when combined in the form of 
neutral salts ; and that decomposition of the soil 
is necessary, either total or partial, in the progress 
of vegetation, is also probable ; in that those meth¬ 
ods of cultivation, particularly burning and freez¬ 
ing, and those manures that possess an anti-acid 
quality, as lime, ashes, &c. and consequently a 
power of decomposing, or at least of lessening the 
adhesive quality of clay, succeed best in producing 
good crops on that soil ; while if used on gravelly 
soils, lime and ashes answer no good purpose, and 
it is even said they are hurtful, which last is a fur¬ 
ther proof that the gravelly soil contains too little 
acid. But I have not yet found any manure, nor 
method of cultivating clay land that produces such 
striking effects as burning. How this effects so 
salutary a change on the clay, I cannot positively 
determine ; but from the above it is at least prob¬ 
able, that it is by diminishing the acid contained ia 
the clay, which if true, might not burned clay,. 
*The manner in which the gypsum is decomposed in this case, 
may be by its meeting some substance for which it may have a 
neater attractiooj and thus decomposes the sqil. 
