ge 
4 
€ 
(438) 
safety-valve v beneath the liquid regulates the out-flow without 
permitting air to be sucked in. 
Into this reservoir may escape also through the tubes S, and Sz, 
the cock A} and the tube S,, the contents of the joining-tube Sj, 
and the mixture of gas and liquid contained in the pump itself. 
The contents of each of the reservoirs S, Dj, Do, which enclose 
gas under pressure, may likewise escape into the reservoir V, or 
directly into the sucking-tube on opening the cocks 3, K; intro- 
duced for that purpose. Moreover the various parts of the arran- 
gement may be connected through the cocks A, and A, to an air- 
pump, to the open air or to a gas reservoir. 
4. Pouring out little quantities of liquid nitrous-oxide. Nitrous- 
oxide is a very important means for operating with low temperatures. 
The boiling point les lower than that of carbon dioxide. It has further 
an advantage over carbon dioxide in remaining liquid at the boiling- 
point. Hence it may be used for transparent (liquid) baths, which 
for most experiments will be preferred to a snow-like substance. In 
physical and chemical laboratories however the free liquid is relatively 
seldom used for this purpose. One of the causes of this may have 
been that, on trying to pour out the liquid directly from the com- 
mercial cylinders, the cock was frozen. Or that, by neglecting to 
sufficiently close the glass-vessel (a vacuum-glass for instance) in 
which the liquid was poured out, part of the N.O, the melting 
point of which lies very near the boiling point was allowed to 
congeal by evaporation in the air!) and so to form a solid mass in 
front of the orifice. However the principal reason will probably have 
been, that in all cases the quantity poured out was very smail in 
proportion to the quantity employed, and hence the price of the free 
liquid thus obtained was far in excess of the moderate price by weight 
of the N90 in the cylinder. 
The N20 may however be esoled easily and with little cost by 
means of carbon dioxide so much, that almost all the nitrous-oxide 
flowing out is received as liquid if the glass into which it is poured 
has been also closed sufficiently. For many experiments then the 
advantages enumerated above will outweigh the smaller cost and 
danger of the solid carbon dioxide, In such cooling experiments the 
gaseous N,O is conducted through a drying tube with P20; (as in 
plate III fig. 5) which is connected with the reservoir of liquid gas, 
to a thick copper condensing spiral (7,5 mm. outer 4 mm. inner 
diameter) consisting of 24 turns, 12 with diameter 8 and 12 with 
1) See NATTERER, Pogg. Ann. 62 p. 184. (1844). 
oD 
