BY THE ADOPTION OF PNEUMATIC RAILWAY'S, 
C3 
piig'lne passes on by its momentum from sta- 
tion to station ; and a contrivance is provided, 
by means of a valve at the stations, by which 
it is brought under the operation of the next 
engine without stopping its motion. 
Although the danger of accidents to passen- 
gers on the present railways worked by loco- 
motive engines, is considerably less than that 
of travelling by horse coaches on turnpike 
roads, yet serious accidents have occasionally 
occurred. These have generally arisen either 
from the locomotive engine running off the 
rails — from one train running against ano- 
ther— from the locomotive engine breaking — 
or, finally, from persons standing upon the 
rails being run down. In the pneumatic 
system there is almost a perfect security from 
these causes of danger. From the engines 
being stationary, and the tunnel rising between 
the wheels of the trains, it is evidently impos- 
sible for the carriages to run off the road ; and 
from the manner in which the system is work - 
ed, it is impossible that one train can run 
against another. It happens also that the na- 
ture of the rails themselves, forming, as they 
do, merely ledges upon the sides of the tunnel, 
prevents the possibility of persons standing 
between or upon them . 
In railways worked by statlonary'engines, 
serious accidents have occasionally occured by 
the ropes breaking while the train has been 
ascending a slope. In such cases the train 
has run down by its weight with a frightful 
rapidity, producing the destruction of the car- 
riages and the loss of life. It is evident that 
this source of danger is removed by the 
pneumatic system. 
An advantage possessed by this system above 
the edge railroad deserves to be particularly 
noticed. In the edge railroad the engines and 
carriages are kept upon the road by flanges or 
ledges raised upon the tires of the wheels 
which press on the interior of the rails. Every 
thing which causes the carriagss to press on 
the one side or the other, causes these 
flanges to rub against the rail. When 
a curve or' bend happens in the road, the carri- 
ages are guided by the pres.sure of one or the 
other flange on the side of the rail, which, of 
course, is accompanied by considerable fric- 
tion. In the pneumatic railway there are no 
flanges, either on the wheels, or rails ; the 
carriages are guided by wheels or rollers 
placed in a horizontal position, and acting upon 
the external sides of the channel which receives 
the valvular cord. By this means all resistance 
which arises from what is called rubbing fric- 
tion is removed, and every surface which moves 
upon another moves upon it with a rolling 
motion. 
(To be continued.) 
ON CALICO-PRINTING. 
By Thomas Thomson, m. d.,f. r.s, l, &e.&c. 
Regius Professor of Chemistry in the 
University of Glasgow^ 
( Continued from page 23 
II — DISCHARGES OF COLOURS. 
—Most colours are fixed to the cloth by mor- 
dants ; or if they be metallic oxides, they 
retain their affinity only that a particular 
state of oxdizement.* Thus madder is fixed 
by alumina, and cochineal by means of oxide 
of tin. Manganese adheres to the cloth only 
when in the state of sesquioxide, and is washed 
away by water the moment it is converted 
into protoxide. Hence, when the printers 
wish to discharge a colour from cloth, they 
employ something that will dissolve the mor- 
dant, or which will deoxidize the oxide, or 
colouring matter, if no mordant be present. 
The dischargers or either acids, or substances 
having a strong affinity for oxygen ; the for- 
mer being employed to dissolve the mordants, 
and the latter to deoxidize the oxides. The 
chief of these are the following : — 
1. CITRIC ACID is much used to dis- 
solve alumina, and peroxide of iron, and thus 
to prevent the formation of colour on particu- 
lar parts of the cloth, by removal of the mor- 
dant, which would otherwise produce them. 
It is obtained by evaporating lemon juice, and 
thickening it with gum-senegal for the 
cylinder, or with gum and pipe-clay for the 
block. Its action is occasionally assisted by 
bisulphate of potash, or sulphuric acid. 
Sometimes the citric acid is first printed on 
white cloth, and afterwards the aluminous or 
iron mordant is applied slightly thickened. 
It is dried immediately to prevent the swell- 
ing of the acid figures. At other times, the 
mordants are first applied, and the acids print- 
ed over them. 
In both cases, the goods are afterwards pas- 
sed through hot water, containing cow dung, 
and well washed before they are dried. This 
removes the mordants from all those parts to 
which the acid has been applied, which of 
course, remains white after the cloth is died. 
2. TARTARIC ACID, thickened with 
gum, is applied by the block, or cylinder, to 
cloth previously dyed Turkey-red, It is then 
passed through an aqueous solution of chloride 
of lime. The acid disengages chlorine from 
the chloride, which of course, destroys the 
colour of those parts to which it had been 
applied, while all the other parts of the cloth 
retain their red colour. When oxide of lead 
is deposited on the cloth, along with the acid, 
and the cloth after passing through the aque- 
ous solution of the bleaching-powder, is pas- 
sed through an aqueous solution of bichro- 
mate of potash. The parts that would have 
remained white, are converted into a fine yel- 
low. This beautiful process is not confined 
to Turkey-red. 
3. PROTOCHLORIDE OF IRON is 
used to discharge the manganese brown, and 
* Almost every thing which can be applied to 
cloth, in a state of solution, and which becomes 
afterwards insoluble in water, either by preci- 
pitation, or spontaneous decomposition, sticks 
to the cloth when it is washed. Water, there- 
fore, does not remove protoxide of Manganese, 
and the protochloride of tin alluded to at the 
conclusion of this section, as a means of removing 
the sesquioxide or peroxide of Manganese, not 
only takes away their oxygen, but converts them 
into a soluble chloride. 
