314 Intelligence and Miscellaneous Articles. 
means of asking for assistance from any point of the open railway, 
without any training beyond that of counting ten slowly and cor- 
rectly. In practice, as between Red-Hill and Reigate, no inconyeni- 
ence or loss of electricity has been suffered from counterbalancing the 
two currents. 
The author states that there are other properties of opposed 
currents to be communicated on another occasion. 
XXXVI. Intelligence and Miscellaneous Articles. 
NOTES ON FROTH. BY DR. GLADSTONE, F.R.S. 
3 "ome liquids, when shaken up with air, form bubbles ; but some 
allow these bubbles to break and disappear the moment they 
are at rest, while others give rise to amore or less permanent froth. 
This difference among liquids appears to be of a specific character, 
and cannot as yet be shown to be dependent on any other quality. 
As a general rule, aqueous solutions of organic bodies are most 
disposed to froth. Beer is a familiar instance. In this case the 
formation of the froth is originally due in a great measure to the 
rising of carbonic acid gas through the liquid; but its permanence 
is totally independent of that, or indeed of any dissolved gas. This 
was proved by subjecting some beer to exhaustion by an air-pump 
till every trace of the carbonic acid and air it contained was removed, 
withdrawing it from the vacuum, and shaking it immediately, when 
a fine froth was produced which was as persistent, or nearly so, as 
if the beer had continued brisk. 
Solutions of the acetates are peculiarly disposed to produce a per- 
manent froth. To such an extent is this the case, that in making use 
of mixtures of salts, I have sometimes distinguished those contain- 
ing an acetate by this property. The acetate of iron is pre-eminent; 
but the acetates of copper, lead, and other metals share this property 
to a considerable extent. Yet acetic acid itself shows no disposition 
to froth; and the bubbles made when alcohol or zther are shaken, 
instantly disappear. Acetate of iron gives off much dissolved air 
when it is exhausted by the pump, but it froths equally afterwards. 
The citrate of iron is analogous to the acetate. 
This property of frothing is quite independent of specific gravity. 
A heavy solution of sulphindigotic acid froths, but a solution of 
chloride of ammonium of high density produces no persistent bub- 
bles when shaken; while, on the other hand, a weak solution of 
soap, that differs little from distilled water, will produce, as every 
one knows, a very permanent froth. 
The froth on the surface of a coloured liquid is always of a lighter 
. tint than the liquid itself. This is what might be anticipated, when 
it is remembered that in looking at froth, we see the light which 
has traversed only very thin films of the liquid, and hence has been 
subjected to but little absorption. Sometimes, however, the colour 
of froth is totally different from that of the liquid on which it floats ; 
cochineal, for instance, gives a deep red aqueous solution, whieh 
