120 
American Milk-Condensing Factories. 
dollars, cents. 
Cost of 2080 gallons of milk at 12^ cents per gallon .. 260 
Daily working expense of factory as before estimated .. 24 50 
4160 tin cases, 3 cents 124 80 
Filling, sealing, labelling cans (1 cent) 40 
Total 449 30 
Prodnct : — 4160 cans of Susared Condensed Milk, at 
29 cents per can .. .. " 1206 40 
Daily balance above expenses 757 10 
The Vaciinm Pan. — In order to show how milk is condensed 
in vacuo it may be well to give an illustration of some of the 
modern improved machinery employed for this purpose. 
Fig. 3, on p. 122, represents an improved cast-iron Vacuum 
Pan. Different liquids, as is well known, boil at different tem- 
peratures, and the same liquid may be made to boil at any 
temperature, from the freezing point up, according as the pressure 
upon its surface is taken off or increased. If by reason of boiling 
in confined space, the pressure upon the surface is increased, so 
that steam cannot readily pass off; the heat accumulates to a 
greater degree than 212°, till the steam acquires sufficient 
elasticity to overcome this increase of pressure. At the bottom 
of deep mines the increased pressure of the air has the same 
effect, and steam is not generated at so low a temperature as at the 
surface. As the pressure is diminished, either mechanically by 
the use of the air-pump, or by ascending elevations, steam is gene- 
rated, and passes off freely, at lower temperatures than 212° F. 
On high mountains it may be difficult to produce sufficient heat 
in open vessels even to boil eggs ; Darwin was led to notice this, 
when he ascended with his sailors one of the mountains of 
Patagonia. They took with them a new pot, in which they 
attempted in vain to boil potatoes. But for the pressure of the 
atmosphere, the ocean would boil and evaporate with heat 
equivalent to that of the sun's rays. Several ingenious experi- 
ments have been devised to illustrate these facts. The simplest 
is in making a glass of warm water boil under the receiver of an 
air-pump. The pulse-glass consists of two glass bulbs, con- 
nected by a glass tube. The fluid in one is made to boil by 
holding one of the bulbs in the warm hand. This property of 
fluids, of being converted into vapour at different temperatures, is 
made to serve important purposes. Liquids intended to be 
evaporated, are sometimes partially freed from the pressure of 
the air, and are thus boiled "in a vacuum" with economy of fuel. 
This process is adopted with great success in sugar refining. 
When the temperature of the usual boiling point would 
injuriously affect the liquid to be evaporated, as milk for instance. 
1 
