TRANSACTIONS OF THE SECTIONS. 45 



to the burner is regulated by two pipes, the cocks of which can be opened and shut 

 from the room as well as the two outside shutters, without opening the sliding win- 

 dows in front. When the operator has to put the plates in the mercury box or to 

 take them out, he has only to stop the external light by shutting the two outside 

 iron shutters and to open the inside glass shutters. The iron closet is then perfectly 

 free from mercurial vapour, and during the sliort time necessary for placing and re- 

 moving the plates no vapour can escape in the room. 



On the Use af a Polygon to ascertain the Intensity of the Light at different 

 angles in the Photographic Room. By A. Claudet. 

 This was of white-coloured wood ; and enabled the operator to ascertain by the 

 appearance of the different facets when DaguciTeotyped, the strength of light and 

 shade from different parts of the room, and so to place the sitter in the best positions, 

 and regulate the light with shades and screens, according to the effect produced on 

 the facets. 



On Agricultural Chemistry, especially in relation to the Mineral Theory of 

 Baron Liebig. By J. B. Lawes and Dr. J. H. Gilbert. 



Mr. Pusey had, in a recent article in the Journal of the Royal Agricultural Society 

 of England, on the progress of agriculture during the last eight years, quoted the 

 experiments of Mr. Lawes and Dr. Gilbert as being conclusive against the " Mineral 

 Theory" of Baron Liebig, which asserts that the crops on the farm rise and fall accord- 

 ing to the supply within the soil of the mineral constituents indicated by an analysis 

 of the ashes of the plant. To these observations of Mr. Pusey, Baron Liebig had re- 

 plied at some length in the new edition of his ' Letters on Chemistry', just published, 

 and in doing so, has asserted that the experiments alluded to are entirely devoid of 

 value, as the foundation for general conclusions, and that the statements of the au- 

 thors could only be made in ignorance of the rationale of agricultural practices on the 

 large scale. 



The authors have therefore given in the present paper an outline of their investi- 

 gations in agricultural chemistry, comprising an extensive series of experiments in 

 the field on the growth of the principal crops, entering into a rotation ; upon the 

 chemistry of the feeding of animals ; and upon that of the functional actions of plants 

 generally, in relation to the soil and atmosphere. And in connexion with all these 

 branches much laboratory labour has constantly been in progress since the commence- 

 ment of the experiments themselves in 1843. The results selected by Mr. Lawes 

 and Dr. Gilbert in justification and illustration of their views, were those of the field 

 experiments on wheat ; which had been grown continuously on a previously exhausted 

 soil for the last eight years, and in each season, by means of many chemical manures 

 by the side always of one or more plots unnianured, and one manured continuously 

 by farm-yard manure. Some of the results thus obtained were illustrated by a dia- 

 gram, from which it appeared that mineral manures had scarcely increased the pro- 

 duce at all when used alone ; whilst the effects of ammoniacal salts were very marked, 

 even when repeated year after year on the same space of ground from which the en- 

 tire crop (corn and straw) had been removed. Indeed, in this way a produce had been 

 obtained even in the sixth and seventh successive j'ears of the experiment, exceeding 

 by nearly two-thirds that from the unmanured plot. It v/as thus shown, that the 

 mineral constituents of the soil continued to be in excess, relatively to the nitrogen 

 available for them from natural sources. The history of several plots was then traced 

 down to the last harvest (1850), and it was argued that the statement assailed by 

 Liebig, viz. that ammonia was especially adapted as a manure for wheat, was fully 

 borne out when speaking of agriculture as generally practised in Great Britain; in 

 other words, that in practice it was the defect of nitrogen rather than of the mineral 

 constituents that fixed the limit to our produce of corn. 



The authors next called attention to the fact of the exhalation of nitrogen by growing 

 plants, as proved by the experiments of De Saussure, Daubeny and Draper, and they 

 referred to some experiments of their own, with the view of showing the probability 

 that there is more of the nitrogen derived from manure given off during the growth 

 of cereal grains, than of leguminous and other crops ; and hence might be explained 

 the great demand for nitrogenous manures observed in the growth of grain. The 



