in its Rrltifioiis to Clicmistri/ and GcoJof/i/. 
223 
as an aildilion to our theoretical probal)ililies ; but it docs not 
afford us alone any sure ground to rest or even safely to speculate 
upon. It suggests still further and more extended inquiries into 
the nature of the circumstances by which this proportion of nitro- 
gen is made to vary, the extent to which the variation takes 
place, and the power of the cultivator to control and benefit by 
it. And so it is with all our best results, they give us a foothold 
by means of which we may take nnolher stride, but rarely a space 
broad enough on which we can stand and rest. 
b. Another opinion in regard to the nitrogen of plants which 
has also had much currency, is, that plants derive all they contain 
from the ammonia of the atmos])here, and that they take it up 
only in the form of ammonia. 'J'his opinion was so contrary to 
the oldest and most common experience of practical men of all 
grades of intelligence in the raising of their crops, that nothing 
but the announcement of it in the form of an undoubted law 
could have secured it any degree of permanent consideration even 
among scientific men. As it was, sanguine young persons, chiefly 
such as were unfamiliar with practice, took it up and warmly 
maintained it both in this country and abroad. A calm con- 
sideration of facts, however, is gradually removing this notion 
from the public mind, and another year or two will banish it 
from our books. 
c. A third opinion adopted by many, and extensively acted 
upon by some, is, that plants obtain all their organic matter 
directly from the air," and derive, and therefore require, only 
mineral matter from the soil.* Contrary as this is also to old 
experience, it obtained a very general acceptation only a few years 
ago. Field experiments were made, and results published, which 
seemed to confirm it.f People began to burn their bones in 
* " The crops on a field diminish or increase in exact proportion to the 
diminution or increase of the mineral substances conveyed to it in manure." 
— LlEHlG. 
f The writers in this Journal were drawn into the vortex, and some of its 
readers will recollect an able prize essay by Mr. John Hannam, in which it 
was believed to be demonstrated by experiment, in especial opposition to 
the opinions stated in my published ' Lectures,' that burned did as much 
good as unburned bones, and that it was the mineral or earthy matter only 
to which their fertilizing efficacy was owing. But experiments, as hitherto 
made and interpreted, maybe used to prove either of two contested opi- 
nions. Like the evidence taken before a committee of inquiry, you find 
the materials in it for making out either case as your feelings incline you, 
though, on carefully comparing the whole, the truth may to unbiassed per- 
sons clearly appear. 
I have recently been informed that experience in Wiltshire is in favour 
of the opinion, that burned bones dissolved in sulphuric acid are more effi- 
cacious than unburned bones so dissolved, and containing the same weight 
of mineral matter. I should be obliged to any practical experimenter wlio 
would send me pKScise numerical results upon this point. 
