converted into two molecules of calcium pentasulphide and three of calcium hydrate.

The fact of the spent lime containing free sulphur might be accounted for in three ways.

(1) As a reaction between CS, and CaS:

CaS+ CS2 = CaS2CS2 + S3.

(2) As a product of the continual action of oxygen on CaS,, in which by equation (i.), or (ii.), or (iii.), CaSO, CaSO or CaSO, might be formed thus:

(i.) CaS, + 0 = CaSO3 + S1.

(ii.) CaS, +01 = CaSO4 + S1.

4

(iii.) CaS,+O3 = CаS2O3 + S3.

(3) As a product of the reaction between CaS, and CO2: CO2+ H2O = CaCO3 + H2S + S4.

CaS,

2

The great secret of success in all systems of purification, is to have ample area in order that the gas may pass slowly through the material; if this is not attended to, the purifying material ceases to absorb before it is spent, and there is an increase of back pressure.

There are various opinions as to the purifying area requisite for efficient working, but a very reliable one is that given by Mr. Newbigging in his excellent "Handbook for Gas Engineers and Managers," p. 143, which is: "Where there is intended to be four purifiers, three always in action, the maximum daily (twenty-four hours) make of gas expressed in thousands, multiplied by the constant 0.6, will give the superficial area in feet of each purifier.”

Gas, by passing through the purifiers loses about three per cent. by volume in the proportion (roughly) of about three-quarters CO, and the remaining quarter SH2.

CHAPTER XI.

THE SIMPLEST METHODS OF ASCERTAINING THE PURITY AND ILLUMINATING POWER of Gas.

THE

HE simplest method of ascertaining the purity of gas is to subject it to certain qualitative tests, which will prove the presence or absence of the impurities, ammonia (NH); sulphuretted hydrogen (SH), and carbonic acid (CO2).

Ammonia is detected by means of a moistened turmeric paper, or reddened litmus paper. Turmeric papers are prepared by adding to one part by weight of turmeric powder, contained in a stoppered bottle, six parts by weight of methylated spirit; the contents of the bottle should stand for about three days, and be well shaken up occasionally during the interval. Some of the clear liquid is then poured out into a photographer's dish, and sheets of filtering or botanical paper soaked therein. These sheets are then hung on lines to dry, cut into strips about two inches long and half an inch wide, and kept for use in a receptacle where they will not be exposed to the action of the light. Ammonia is detected by exposing one of these moistened papers to a stream of gas. If the gas contains this impurity, the colour of the paper will change to a brownish tint, and if much NH, be present, to a deep red colour. The presence of so small a quantity as about one grain of ammonia in 100 cubic feet of gas will cause a change in the colour of the test paper. The papers should

be of a full yellow colour, but not so decided as to approach an orange tint, which would render them less sensitive.

Ammonia may also be detected by means of a reddened litmus paper, which turns blue in the presence of ammonia.

Reddened litmus papers are prepared from blue litmus in the following manner:-In a stoppered bottle place half an ounce of powdered litmus, and to this add three ounces of cold distilled water, the whole should be well shaken, and then left, so that the litmus may dissolve. The extracted litmus is then filtered off, and to it is added, drop by drop, a dilute solution of sulphuric acid, until a distinct red tint appears. Strips of glazed paper are then drawn through the solution, and hung up to dry in a room free from acid fumes. Ordinary glazed paper produces a more sensitive test paper than filter paper does.

Sulphuretted hydrogen is detected by means of a paper coated with acetate of lead or nitrate of silver.

Test papers of this description are made by moistening sheets of filter paper with a solution of one part of sugar of lead in eight or nine parts of water, and holding each sheet, while still damp, over the surface of a strong solution of ammonia for a few moments. As the paper dries, all free ammonia escapes. Such papers on being exposed to a stream of gas containing sulphuretted hydrogen, turn a brownish-black tint, owing to the formation of sulphide of

lead.

Carbonic acid is detected by means of a solution of lime or baryta water, generally the former.

Lime water is prepared by placing about four ounces of caustic lime into a bottle containing a quart of water, allowing it time to dissolve, and shaking it occasionally during this operation. The lime is then allowed to settle, and the clear liquid transferred to another bottle, which should be kept securely corked, so as to prevent the access of CO2. In order to make a test, about an ounce of the

solution is placed in a test tube, and the gas caused to bubble through it by means of a fine glass tube connected to the gas supply by a piece of indiarubber tubing. Should there be any CO2 present, this would combine with the lime in the lime water to form a white precipitate of calcium carbonate, thus: CaO + CO, CaCO3. If no precipitate forms during a period of three minutes, we may assume that the gas is free from CO2.

=

It is necessary to note when testing for CO2 in the presence of SH2, that the latter impurity must be removed before the gas enters the lime water, otherwise the results will be misleading. The SH, may be kept back by means of a small oxide purifier.

The simplest method of ascertaining the illuminating. of gas is by means of an instrument known as the 'Jet Photometer."

power

The jet photometer is really a specific gravity test, and is based on the fact that the length of a gas jet issuing from an orifice of a certain size, under exactly uniform pressure, varies according to the illuminating power of the gas, the richer the gas the longer will be the flame; or if the length of flame be kept constant, the richer the gas, the less will be the pressure required to give this length of flame.

The instrument consists of a delicate King's gauge, surmounted by a steatite jet having a fine orifice.

In front of, and behind, this jet are glass plates, graduated upwards from the level of the burner top into inches and tenths of an inch. The photometer is inclosed in a glazed wooden case, provided at the top with a glass chimney. On one side of the case two dry governors are fixed, through both of which the gas passes, the second one being weighted to give a less pressure than the first.

From the outlet of the second governor, the gas passes through a pipe provided with a regulating cock to the

K

King's gauge. The gas pressure, by means of these appliances, is readily adjusted, and regularly maintained. On the opposite side of the instrument a water cistern is fixed, and by a suitable contrivance water may be admitted into or withdrawn from the King's gauge.

A jet photometer, as described above, is shown in fig. 31, and supposing that the instrument has to be put to work for the first time, the following are the instructions to be observed:

1

"Set the instrument on a firm and level base or support; carefully remove the float and the packing paper from the cylinder. Replace the float in the cylinder so that its cord lies in the front groove of the wheel above it. Pass the cord of the weight round the back groove of the wheel, and let the weight hang in opposition to the float. Take care not to strain the cords. Attach a gas supply pipe to the inlet of the governor. Pour water gently into the cylinder until the pointer exactly reaches the zero of the pressure scale. If too much water be put in, draw off the necessary quantity from the side cock. Turn on the gas, ignite it at the jet, and apply a sufficient number of shot, or of pieces of metal, to the second diaphragm of the double governor to cause the gauge to indicate, say, eight or ninetenths pressure. Put the glass scales in their position, behind and before the burner, and make any further reduction which may be needed in pressure by the aid of the cock, or valve, at the side of the instrument. Readjust the waterlevel once a day."

In order to get some idea as to the value of the flame length when compared with the actual illuminating power, it is necessary to "rate" the instrument, that is, to determine the length of flame corresponding to a certain quality of gas under a certain pressure, or keeping the flame

Hartley's "Gas Analysts' Manual."