iSl6.] Scientific Intelligence. S17 
leaves, but have coalesced into a sort of trunk 10 feet high, and 
pretty thick, which is very singular in being now terminated by 
roots both at top and bottom.—This curious fact is related in a letter 
by Dr. Walker, late Professor of Natural History in the University 
of Edinburgh, to Lord Kames, dated Feb. 18, 1773, published in, 
the Memoirs of the Life and Writings of Lord Kames by Lord « 
Woodhouslee, vol. iii. p. 207. 
Ill. Depth at which Barley will grow. 
Dr. Walker, in the same letter, states, from his own observa- 
tion, that barley will rise though sown to the depth of 10 inches, 
but will not rise if placed’12 inches deep. ‘This depending upon 
the access of air to the grain, shows us how far under the surface ot 
the earth the air is capable of penetrating. 
IV. On burning Clay as a Manure. 
(To Dr. Thomson.) 
DEAR SIR, Dumfries, Feb. 19, 1816. 
Being one of the constant readers of your Annals of Philosophy, 
and looking at your title-page, 1 see you give all agricultural sub- 
jects an insertion in your publication. Situated as | am, in a part 
of Scotland where agriculture is quite in its infancy, and where the 
application of chemistry to the improvement of that most useful 
branch of national industry is altogether unknown, I take the 
liberty of submitting the following observations to your considera- 
tion, hoping your kindness to an ignoramus may overlook any im- 
pertinent questions J am now about to put. All this country trom 
Port Patrick to almost the eastern extremity of Scotland, is under- 
going the operation of clay-burning. It is natural to ask what is 
the cause of all this, and what benefit, or what alteration, does the 
clayey mass undergo by this torrefaction by fire? Many times have 
I inquired from some of these great clay-burners, but could never 
obtain any explanation, as unfortunately none of these Gentlemen 
are at all acquainted with the science of chemistry. ‘I can under- 
stand so far, that where the clay to be burnt contains avy portion 
of calcareous matter in it, the operation of the fire may convert it 
into a caustic state, and by that means convert it into an active 
manure ; also the carbonaceous matter used in firing the clay may, 
by having its ashes mixed with the burnt clay, be useful as a com- 
post for various plants, as it has been ascertained that carbon is the 
food of plants. Some clay, also, may contain much sulphate of 
iron, which may sometimes, by an excess of acid, be injurious to 
vegetation ; and the carbonaceous matter may, when used in burn- 
ing the clay, tend to dispel the sulphuric acid, and leave behind the 
iron, which may absorb oxygen, and afford it in quantity to be 
‘absorbed by the roots of plants. I believe that iron is capable of 
absorbing various doses of oxygen, but 1 do not know whether it 
has been experimentally tried if the roots of plants can absorb the 
oxygen from the metal. I have heard some assert that oxide of 
