• When the first part of the retort is red-hot and the escape of air from expansion has ceased, about an inch more of it may be heated, and so the fire gradually carried down; about three bubbles of air may pa.ss in two seconds, it is better not to attempt a more rapid disengagement. At first but a small portion of the gas is absorbed, but when the substance is fairly undergoing decomposition, and the atmospheric air in the apparatus has been expelled, It is almost entirely taken up by the potash-ley. When the whole tube is ig nited the heat must be continued till bubbles are no longer disengaged ; the potash-ley will now gradually recede into the large bulb ; when this is observed to commence, the char coal rnust be removed from the tail of the tube; and as soon as the potash has risen to fill half the large bulb, the tip of the tail must be nipped off; and over the opened extremity a tube about eighteen inches long, and one quarter in diameter, should be supported; gentle suc tion is then effected by a sucking tube (fi,g. 430, D) fitted to the free extremity of the potash apparatus, drawing air through the combustion tube to displace the carbonic acid and aqueous vapour it contains. The use of the long tube over the end of the retort is to supply pure air, and to prevent that from the furnace charged with carbonic acid from passing freely into the apparatus: the actual process of cotnbus tion performed in the manner above described usually occupies from an hour to an hour and a half.
The plan of drawing air through the tube is that practised by Liebig, and it admits of con siderable accuracy. Dumas, however, con nects the extremity of the retort with a drying tube, and this again with a receiver containing oxygen, which gas is carefully driven over the contents of the tube. This renders the ope ration somewhat more complicated, but it is unquestionably more exact, especially in com pounds where the proportion of carbon is great. The tube for supplying oxygen is easily ad justed to the retort by drawing out the tail horizontally instead of obliquely, and fitting it on by a caoutchouc connector, care being taken to screen the junction from the influence of heat.
The apparatus is now dismounted, and t whole allowed to cool; in about an hour t drying tube may be weighed, and the incre of weight carefully noted ; one-ninth of t gain indicates the quantity of hydrogen t substance contained ; the potash apparatus also weighed, and three-elevenths of what has gained shews the quantity of carbon. T deficiency is oxygen.
The oxide of copper used in these ex riments may again be rendered serviceable moistening it with nitric acid, and igniting before.
2. Analysis of a liquid not containing trogen.
If the fluid be volatile, we take a piece o tube rather less than a quarter of an inch dia rneter, heat it in the blowpipe flame, and draw on it a capillary portion about four inches long; about a quarter of an inch below this the tube is sealed ; the little piece of tube thus left connected with the capillary part is heated and blown into a small bulb about as big as a good-sized pea; this is cut off', leaving a ca pillary neck of about two inches long. Having
made a sufficient number of these little bulbs, we take two of them, which we have ascer tained will freely enter the combustion-tube, and weigh them accurately; a little of the liquid to be analysed is put into a small tube, and tlit capillary neck of the bulbs inverted into the liquid; the bulbs are then warmed by the flame of a spirit-larnp, so that on cooling they shall be about three-fourths filled with the liquid. The necks are now sealed by the blowpipe flame, and the bulbs again weighed ; the in crease of weight gives the quantity of the liquid which has entered, and which is to be analysed. The oxide of copper having been heated, and allowed to cool with the usual pre cautions, about an inch and a half of the retort is filled with pure oxide ; we then take one of the bulbs, draw a file across the capillary neck, put the bulb into the tube, break off the neck by pressure ag,ainst the glass, and drop the broken portion in with the bulb; we then pour in a couple of inches of oxide of copper, introduce the second bulb in the same manner as the first, and fill up the tube with oxide, cork it and strike it smartly on the table as before to secure free air-way. The combustion tube is now connected with the exhausting syringe, but no heat applied; on gently working the syringe, the air and vapour in the bulbs will expand and drive out the liquid, which will be quickly absorbed by the oxide of copper around ; we adjust the apparatus in the furnace as before, and gradually heat the upper half of the tube; when this is red, we vola tilize the fluid by cautiously approximating a piece of ignited charcoal, taking especial care not to heat the tube too much ; by degrees all the liquid in the first bulb is expelled, and we proceed in like manner with the other ; the whole tube is finally heated carefully, and the after part of the process conducted in the manner already described.
If the liquid he not volatile, an oily acid for example, a small vessel is made by taking a piece of glass tube about a quarter of an inch in diameter, sealing one end, and while hot press ing it on a flat surface, so as to make a firm basis on which it may stand upright ; it should be cut off the tube so that the little vessel be about an inch high. It is weighed at first empty, and a second time with the liquid for analysis; an inch or two of oxide of copper is tput into the combustion-tube, then the vessel with the liquid. On inclining. the tube suffi ciently, the liquid runs out and is made to diffuse itself over the inner surface of the lower half of the tube, which is to be filled with oxide of copper, and the analysis to be cautiously proceeded with in the usual way.