48 REPORT—1857. 
water into rhodizonate of potash, a substance known to be explosive. The specimen 
contained no rhodizonate ready formed. 
2. On Froth.—Some liquids when shaken with air form a more or less permanent 
froth. Aqueous solutions of organic bodies are peculiarly disposed to do so. ‘The 
frothing of beer is due originally in a great measure to the carbonic acid that rises 
through the liquid, but its persistence is quite independent of that or any other dis- 
solved gas, as was proved by exhausting some beer by an air-pump, and afterwards 
shaking it. Acetates are much given to making a permanent froth when dissolved 
in water, whether the solution contain air or not; yet acetic acid itself is not remark- 
able for this quality, and alcohol or zther forms bubbles when shaken which instantly 
disappear. The power of producing a persistent froth appears to be a specific quality 
not depending on the density of the liquid or any other known property. The colour 
of froth is always lighter than that of the liquid from which it is produced, and in 
some cases it is totally different. The author showed that this was due to the dichro- 
matism of such liquids; for instance, a thin stratum of cochineal transmits rays 
which are absorbed by a larger quantity of the substance. In a similar manner the 
colourless bubble that floats on port wine was explained by a prismatic analysis. 
On the Decomposition by Heat of certain Ammoniacal Salts. 
By J. H. Grapstone, Ph.D., F.RS. 
The author showed that the decomposition of phosphate and sulphate of ammonia by 
astrong heat was not entirely owing to the non-volatility, unless at a very high tempera- 
ture, of phosphoric and sulphuric acids. In fact these salts are decomposed partially 
when their solutions are boiled, ammonia being given off, and the remaining liquor 
becoming acid. In like manner oxalate of ammonia is capable of decomposition, and 
crystals of the citrate give off the volatile alkali even at the ordinary temperature, . 
acquiring at the same time an acid reaction. It was noticed that the ammonia salts 
of the monobasic hydrochloric and nitric acids are not decomposable by water, while 
the compounds of the bibasic, oxalic, and sulphuric acids are liable to partial decom- 
position, and those of the tribasic, phosphoric, and citric acids are still more easily 
resolved into free ammonia and acid salts. 
On the Use of the Prism in detecting Impurities. 
By J. HW. Giapstone, Ph.D., F.RS. 
This paper described the novel use of the prism in detecting impurities. The author 
described the methods of examining substances by means of a prism, especially the 
instructive results obtained with liquids when the ray of light traverses them in a 
wedge-shaped vessel. He suggested this as a means of detecting coloured impurities 
when they do exist, and of proving their absence when they are wrongfully suspected. 
He showed the value of the means in respect to coloured confectionery, tea, and mus- 
tard, and remarked on its use in examining wines, liqueurs, pigments used in the fine 
arts, gems, pharmaceutical preparations, &c. He stated that the prism and hollow 
wedge were already used as a commercial means of ascertaining the purity of certain 
substances, 
On Electrical Currents in the Earth's Surface. 
By ArcuisaLtp H. Hamiiton. 
In the spring of the present year, the author had occasion to try a series of experi- 
ments on a difficult point, namely, the nature of the earth as a conducting body. 
Having selected six convenient stations, represented in the diagram, he buried dif- 
ferent metallic bodies in those marked A, B, C, D, and wooden boxes, filled with 
water, in which metal plates were to be plunged, at stations No. 1 and No. 2; these 
stations were connected by wires sufficiently insulated to convey currents of a single 
cell, without sensible loss in ordinary weather. 
On the evening of April 20, 1857, about 6:30 p.m., he proceeded to make some 
observations with a small galvanometer. He first connected a brass plate in box No. 1 
with the brasses buried at A and B, and found a strong deflection, arising as it were 
from zinc plates at A and B. He then took down the galvanometer to C, and con- — 
nected by its wire the plates buried at C with a zinc plate plunged in No. 2, and — 
