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NATURE 
[ Fed. 1, 1883 
soluble and energetic than the same amount contained in porous 
phosphatic materials, such as certain descriptions of phospho- 
guano. 
6. Treatment with acids renders the material completely 
soluble in water, and the so-formed superphosphate, when put 
into the ground, is precipitated in a very fine state of division. 
7. Inthe precipitated state the insoluble phosphate is im- 
measurably more finely divided than it could be obtained by 
mechanical means, and is consequently more energetic than any 
raw material mechanically ground, 
8. The anthor’s conclusion is that the chemical treatment with 
acid is the cheapest and best way of rendering mineral phosphates 
useful for agricultural purposes. 
We think that it will be generally admitted that these proposi- 
tions give a very reasonable statement of the case ; but for the 
purposes of our inquiry we must supplement them with the fol- 
lowing additional proposition. This has reference to a matter 
which has escaped the attention of Dr. Voelcker, but which is 
strongly supported by the results of the numerous recently- 
recorded practical trials. 
‘* By reprecipitating the acid in a super-phosphate previous to 
its employment for agriculture by means of a suitable base, it 
becomes possible to obtain a neutral phosphate, possessed of a 
sufficient degree of solubility to be readily distributed through 
the soil, in an extremely fine state of subdivision, and capable of 
affording nutriment to the plant under highly favourable 
conditions.” 
It is to this further proposition to which we now desire to call 
special attention, and we may allude first to the assumed loss of 
the power of spontaneous diffusion through the soil, which is 
stated by Sibson, in his work on ‘‘ Artificial Manures,” to 
render the precipitated phosphates inferior in value to soluble acid 
phosphate. We think that no chemist will doubt that the phos- 
phates in guano are sufficiently soluble to be available for plant 
food, and precipitated phosphate is certainly more soluble than 
the earthy pho=phates in Peruvian guano. It must be remem- 
bered, moreover, when studying the table of solubilities of 
phosphates, as ascertained by Dr. Voelcker, that these are stated 
with reference to distilled water, which does not occur in nature, 
whereas in water containing small percentages of many of the 
salts, commonly present in the soil, the solubility of phosphates 
is largely increased. Thus the addition to the water ofa trifling 
amount of ammonic chloride (1 per cent.) increases the solubility 
of precipitated calcic phosphate fourfold. 
This matter has not then received a due share of attention, for, 
as we have seen, arguing on the analogy of guano, phosphates, 
in the precipitated form, are undoubtedly so far soluble as to 
possess the power of diffusion to an extent amply sufficient for 
agricultural purposes, and there must be a point, short of perfect 
solubility, which adequately satisfies all requirements in this 
respect. A careful consideration of the subject has led us to the 
conclusion that the effect of phosphoric acid added to the soil, 
after having been fixed by a suitable base, in a condition suffi- 
ciently soluble for every need of the plant, and in a state of sub- 
division far finer than anything which could be obtained by 
mechanical means, would be in theory, if not superior, at least equal 
to that of a similar amount of soluble phosphate, applied to a soil 
promiscuously, in casesin which it is impossible to predict by 
what bases the phosphoric acid will be fixed, or even whether it 
will be fixed at all. Indeed, the foregoing considerations would 
almost lead us to the belief that the employment of such ready- 
formed compounds as calcic or magnesic phosphates would be 
preferable tothe haphazard use of soluble phosphoric acid in a 
super- phosphate. 
Chemists in treating of the magnesic | hosphates appear to have 
overlooked the dibasic phosphate and to have conducted their 
experiments and to have founded their observations mainly, if 
not entirely, on the behaviour of the far less soluble tribasic 
phosphate. The freshly-precipitated magnesic phosphate is 
soluble in about 322 times its weight of pure water, while 
calcic phosphate, as we have seen when newly precipitated, is 
soluble to the extent of 5°56 grains per gallon. Both of these 
salts are therefore much more soluble than the earthy phosphates 
present in guano. We must not overlook the fact also that 
although it has not yet received much attention, the magnesia 
would appear to possess in itself considerable manurial value. 
Aj}recent French authority assigns to it a value approaching 
5s. 8@. per unit, almost three-fourths of the price he sets down 
for phosphoric acid, and we are convinced from the study of the 
composition of numerous fertile soils, the ashes of plants, and 
recent field-trials, that the day is not far distant when the 
magnesia will rank as high in a manure as a salt of potash, 
Another fact which the foregoing considerations have forcibly 
brought before us is the value of organic matters, in bringing 
about the solubility ot the phosphates. This is perhaps scarcely 
within our present scope, but we have mentioned, incidentally, 
that small quantities of ammonia and carbonic acid, dissolved in 
the water, produce a very marked effect on the solubillty of the 
phosphates. So valuable is their office in this respect, that it 
seems a false system to deny that organic matter, when present 
in a manure, posse ses any value whatever. It was formerly the 
practice with agricultural chemists to allow 1/. per ton (2"4d. 
per unit) for organic matrer, and we think that the important 
office which it fulfils in supplying carbonic acid for bringing into 
solution additional quantities of the phosphates, fully justifies 
the assignment to it of the above valuation. 
We have thus endeayoured to explain the true conditions 
under which phosphoric acid becomes, in the soil, a source of 
plant-food. We have shown that there must be a limit to the 
value of solubility, merely considered as a means of securing dif- 
fusion through the soil, because partially soluble salts also possess 
the property to a devree sufficient for all practical purposes. In 
conclusion we have claimed for a ready-formed, partially- 
soluble phesphate, in a finely divided condition, and, in the 
case of the magnesic phosphate, possessing the property of 
fixing at the same time a portion of the ammonia, a value at 
least as great as that of a soluble acid phosphate, which runs 
the risk of being fixed by iron, or alumina (should lime be 
deficient in the soil), or, which may sink below the roots of the 
plan's before it is neutralised. We trust we have thus shown a 
good case fora more liberai valuation of precipitated phosphates, 
and have indicated, with some measure of success, the reasons for 
the excellent results that have been recently obtained by the use 
of manures containing phosphoric acid in this form. 
THE BLECTROLYTIG BALANCE Ors 
CHEMICAL CORROSION * 
THs paper treats of some fundamental points in silver electro- 
plating, and shows how a large amount of the electric 
power may be wasted by the use of too large a proportion ot 
free potassic cyanide in the plating solution, or by using the 
liquid in a heated state. 
In it is also described a method of ascertaining the degree 
of energy of chemical corrosion of metals in electrolytes, by 
means of the strength of electric current per unit of surface 
necessary to prevent such corrosion; the metals and liquids 
employed for the purpo e in the present research being silver, 
and solutions of argento cyanide of potassium containing free 
potassic cyanide. Numerous examples, chiefly in the form of 
tables, are given of the strength of current required to enter 
cathodes of a given amount of surface, in order to exactly 
balance the chemical corrosive effect upon them at atmospheric 
temperatures, and at hizher ones, of solutions of potassic cyanide 
of various degrees of strength. 
The method employed was to take a given solution of cyanide 
of potassium, pass through it by means of a sheet of platinum 
anode and a burnished sheet of silver cathode, a weak electric 
current, and add gradually to the liquid (with stirring) small 
portions of argento potassic cyanide, until the faintest percepti- 
ble deposit of silver occurred. The verge of deposition thus 
attained was called ‘‘the balance point ;” and the conditions 
which determine and influence it, constitute the subject of this 
research, 
The effect of various conditions upon the point of balance 
of electric and chemical energy were investigated, and the 
experiments are described. The influences examined were: 
composition of the liquid, strength of current, size of cathode 
and density of current, electro-motive force, temperature, 
ordinary chemical corrosion, nature of the cathode, etc. The 
circumstances were also investigated which affect the measurement 
of the current by the method employed in this research, viz. by 
depo iting silver from a solution of argento potassic cyanide ; 
and the sources of error, (and their limit), in that method, are 
pointed out. The effect of varying the proportions of free 
potassic cyanide, and of argento potassic cyanide, upon the 
strength of current at the balance point, are shown in tables of 
results. The strengths of current just sufficient to prevent all 
X Abstract of paper by G. Gore LL.D., F.R.S., read before the Birming- 
ham Philosophical Society, Dec. 14, 1882. 
