64 REPORT—1850. 
stance is found in many natural waters, percolating rocks which contain phosphoric 
acid. Such waters therefore may be applied with advantage for irrigation. The ad- 
vantages derived from this too-often neglected natural source are strikingly exhibited in 
the irrigated meadows in the neighbourhood of Cirencester ; and it is the opinion of the 
author, that one of the chief causes of the beneficial effects which follow the applica- 
tion of the water for irrigation in this locality is to be found in the phosphate of lime 
it contains. In a tea-kettle incrustation, formed in a short period by this water, the 
proportion of phosphoric acid was found to amount to 125 per cent., showing a con- 
siderable quantity of this acid present in the water. A very hard water from Edin- 
burgh was likewise proved to contain phosphoric acid; but its proportion was not so 
large as that in the Cirencester water, the quantity of phosphoric acid in a boiler-in- 
crustation formed by this Edinburgh water being only 0°427 per cent. Sea-water also 
contains phosphoric acid, but the proportion of the latter amounts to mere traces. A 
quantitative determination of phosphoric acid in the boiler-deposit of a Canada steamer 
gave only 0:0306 per cent., and that in a boiler incrustation of a steamer plying be- 
tween Dublin and Liverpool 0°0424, as the per-centage of phosphoric acid. In con- 
clusion, the author recommended Svanberg’s test, molybdate of ammonia, as a ready 
means for detecting the presence of phosphoric acid in natural waters. 
On the Per-centage of Nitrogen as an Index to the Nutritive Value of Food. 
By Dr. A. VoELCKER. 
The object of this paper was to show, that the usual estimation of the nutritive 
qualities of un article of food is frequently attended with inaccuracies, which render 
it desirable to modify our present methods in this respect in many cases. A cireum- 
stance which leads to considerable errors, is the presence of ammoniacal salts in the 
juices of plants. Having examined several vegetables, and constantly found a con- 
siderable amount of ammoniacal salts in the sugary extracts of the juices of plants, 
from which all soluble proteine-compounds had been previously separated, the author 
inferred that the nutritive value of various vegetables had been overrated, as the 
amount of nitrogen they furnished on combustion with soda-lime was taken as the 
index to their nutritive value. Ifthe whole amount of nitrogen in juicy vegetables 
is assumed to exist in the plant in the form of albuminous substances, and calculated 
accordingly, a considerable excess will be found in summing up the results of the 
analysis; which proves indirectly that the whole of the nitrogen in vegetables is not 
contained in them in a form in which it can be considered to add to the nutritiveness 
of the vegetable. In cauliflower, and in the leaves of the same plant, an excess of 
nearly 12 per cent. was experienced in the proximate analysis, by calculating the 
per-centage of proteine-compounds from the amount of nitrogen obtained by direct 
combustion. 
In order to prove experimentally the presence of ammoniacal salts in larger quan- 
tities than hitherto suspected, and to avoid the objection that they might result from 
a partial decomposition of albuminous substances during the analysis, the author 
chose fungi for his experiments, which are rich in nitrogen, and known as being 
highly nutritious, and which, growing abundantly on the Cirencester College grounds, 
immediately adjoining the laboratory, could easily be got in the perfectly-fresh state. 
The species used was Agaricus prunellus, a species which is edible, and remarkable 
for forming most beautiful fairy-rings. After having separated all soluble proteine- 
compounds by means of basic acetate of lead, which reagent throws down these com- 
pletely, the amount of nitrogen still present in the juice of these Agarics, in the form 
of ammoniacal salts, was found to be 0:204 per cent. for the fresh fungi, or 1°82 per 
cent. for the dry fungi. 
The whole amount of nitrogen in the same Agarics, collected at the same time, 
determined by combustion with soda-lime, was found to be 0°74 per cent. for the 
fresh fungi, or 6°61 per cent. for the fungi dried at 212° F. Deducting from the last- 
stated numbers the quantity of nitrogen found to exist in the juice in the form of am- 
monia, we find that only 0°536 per cent. of nitrogen in the fresh, or 4:79 per cent. of 
nitrogen in the dry fungi, exists in the state of proteine-compounds, and that nearly 
one-third of the nitrogen obtained by direct combustion exists in the form of am- 
monia in the juice, or at all events in a form in which the nitrogen adds nothing to 
