cock attach by a good cork the neck of a tubulated receiver of 100 or 150 c. c. capacity, the interior of which has been loosely stuffed with cotton. Into the second tubulure of the receiver fit tightly the delivery-tube carrying a caoutchouc connector; into this connector can be fitted a tube adapted to convey the gas in any desired direction. When many persons use the same generator, each person must bring his own tube.

To charge the apparatus, fill the cylinder with dilute acid to within 10 or 12 c. m. of the top, fill the basket with fragments of sulphide of iron, hang the basket in the bell, and put the bell-glass full of air into its place with the stop-cock closed. On opening the cock, the weight of the acid expels the air from the bell, the acid comes in contact with the solid in the basket, and a steady supply of gas is generated until either the acid is saturated or the solid dissolved; if the cock be closed, the gas accumulates in the bell, and pushes the acid below the basket so that all action ceases. In cold weather the apparatus must be kept in a warm place. For generating sulphuretted hydrogen, sulphuric acid diluted with fourteen parts of water is used; for hydrogen, zinc and sulphuric acid diluted with four or five parts of water; while for carbonic acid, chalk and muriatic acid diluted with two or three parts of water, should be taken.

91. Mortars. Whenever the substance to be analyzed occurs in the form of large pieces or coarse powder, it should, as a general rule, be pulverized by mechanical means before subjecting it to the action of solvents. Mortars of iron, steel, agate or porcelain are used for this purpose, according to the character of the substance to be powdered.

An iron mortar is useful for coarse work and for effecting the first rough breaking up of substances which are subsequently powdered in the agate or porcelain mortar. If there be any risk of fragments being thrown out of the mortar, it should be covered with a cloth or piece of stiff paper, having a hole in the middle through which the pestle may be passed. Instead of the common iron mortar, a small steel mortar, of the kind called diamond mortars by dealers in chemical ware, may be used for crushing minerals. Pieces of stone, minerals, and lumps of brittle metals may be safely broken into fragments suitable for the mortar by wrapping them in strong paper, laying them so enclosed upon an anvil and striking them with a heavy hammer. The paper envelope retains the broken particles which might otherwise fly about in a dangerous manner, and be lost.

The best porcelain mortars are those known by the name of

xlvi

MORTARS. — SPATULE.

Wedgewood-ware, but there are many cheaper subs lain mortars will not bear sharp and heavy blows; th rather for grinding or triturating saline cubstance mering; the pestle may either be formed of one piec or a piece of porcelain cemented to a wooden handle the less desirable form of pestle. Unglazed porcela to be preferred. In selecting mortars, the following be attended to, 1st, the mortar should not be po not to absorb strong acids or any colored fluid, even be allowed to stand for hours in the mortar; 2d, it s hard, and its pestle should be of the same hardness; be sound; 4th, it should have a lip for the convenien out liquids and fine powders. As a rule, porcelain mo endure sudden changes of temperature. They may b rubbing in them a little sand soaked in nitric or sulp if acids are not appropriate, in caustic soda.

Agate mortars are only intended for trituration; a break them. They are exceedingly hard, and impern material is so precious and so hard to work, that agate always small. The pestles are generally inconvenientl difficulty which may be remedied by fitting the agate wooden handle.

In all grinding operations in mortars, whether of p agate, it is expedient to put only a small quantity of the to be powdered into the mortar at once. The operation ing will be facilitated by sifting the matter as fast as it is returning to the mortar the particles which are too lar through the sieve.

92. Spatulæ. - For transferring substances in pow small grains or crystals, from one vessel to another, sp scoops made of horn or bone are convenient tools. A co paper-knife makes a good spatula for laboratory use. from glaze and enamel, are excellent substitutes for spatul

NOTE. In the place of the "Bunsen pump " mentione and 75,may be used, where there is sufficient head of wate aspirator" described by Professor Richards in the "The Journal of Science," VIII (1874), p. 412. The laboratori Institute of Technology and some other laboratories are nished with such apparatus (made of brass, by Thomas F. 101 Portland St., Boston), both for ordinary filtration and ing the blast-lamps.