186 The N.Z. Journal of Science and Technology. [May 
extreme cases to take up the whole space previously occupied by the liquid 
as well, so that there is no liquid, as such, left.” Foaming in a steam boiler 
. causes water to be drawn over with the steam into the cylinders of the 
engine, thus producing what is known as “ priming.” Both foaming and 
priming are highly dangerous. 
In the bulletin the generally accepted causes of foaming are reviewed, 
and theoretical considerations discussed at length. Numerous experiments 
are described, and analytical results tabulated. 
The principal conclusions drawn are— 
(1.) That the salts which produce foaming may be divided into two 
classes, active and passive. The active salts are magnesium car¬ 
bonate, calcium carbonate, sodium carbonate. These actually 
bring about the foaming. The passive salts are magnesium sul¬ 
phate, sodium chloride, sodium sulphate, magnesium chloride, 
calcium chloride, calcium sulphate. These materially assist 
foaming by bringing about the necessary condition, and may 
be regarded as the salts which bring about the foaming when 
they are excited by the active salts. 
(2.) While the readiness with which foam will form depends on the 
concentration of the passive salts, it is almost independent of the 
concentration of the active salts. 
(3.) Organic matter in solution may cause foaming. 
A laboratory foam test is described which is said to have given very 
good results. 140 c.c. of the water to be tested are placed in a 400 c.c. 
tall beaker, of resistance glass, and without a lip. The beaker is about 
12*5 c.m. high and 7 c.m. diameter. It must be quite clean, and especially 
free from grease and acid. It is covered with a watch-glass, and placed on 
a piece of wire gauze on a triangle, or, better, on a Davies furnace. A very 
hot Bunsen flame is applied, and the water is vigorously boiled until the 
bulk is reduced to 70 c.c. The water is then filtered while hot into another 
clean beaker of the same dimensions. The beaker is covered with a watch- 
glass, placed on the flame, and the water again vigorously boiled until the 
bulk is reduced to 5 c.c. If the water in the beaker foams before the filtra¬ 
tion stage is reached it is not suitable for locomotive use. -If it foams after 
filtration it will have a tendency to produce priming, and that tendency 
will be the more marked the sooner the foaming is noticed after filtration. 
If foaming occurs only when the water is taken to very low bulk there will 
be no trouble if the boiler is washed out frequently and undue concentration 
prevented. 
One practical result is a method of preventing foam by treatment of a 
water with acid. The water should first be softened, and after removal of 
the sludge made neutral, or faintly acid, with sulphuric acid. When foam¬ 
ing is due to the presence of organic matter it must be removed before the 
acid treatment is applied. The boiler in which such water is used should 
be free from scale. W. D„ 
Investigations regarding Heat-insulating Materials, by B. J. Smart 
and P. Pecover. Commonwealth Engineer, vol. 5, pp. 127-32, Dec. 1, 
1917. 
In a country so dependent on refrigeration as, New Zealand this is a 
subject of vital importance. It has been investigated by many experi¬ 
menters. The latest is a full research made in Australia by Messrs. B. J. 
Smart and P. Pecover. They adopt as a practical working-unit of heat- 
conducivity of insulating-materials the number of British thermal units 
which pass in one hour through a layer of material one inch thick, having 
