UJTBODUCWON ( J 



substance is also obtain* 'd by heating copper to redness in air that is, 

 it is the scoria or oxidation product of copper. The weight of the 

 black oxide of copper left is less than that of the copper carbonate 

 originally taken, and therefore we consider the reaction which occurred 

 to have been one of decomposition, and that by it something was sepa- 

 rated from the green copper carbonate, and in fact by closing the orifice 

 of the vessel in which the copper carbonate is heated with a well- 

 litting cork, through which a gas delivery tube 15 passes whose end is 

 immersed under water, it will be observed that on heating, a gas is 

 formed which bubbles through the water. This gas can be easily 

 collected, as will presently be described, and it will be found to essen- 

 tially differ from air in many respects ; for instance, a burning taper 

 is extinguished in it as if it had been plunged into water. If weighing 

 had not proved to us that some substance had been separated, the 

 formation of the gas might easily have escaped our notice, for it is 

 colourless and transparent like air, and is therefore evolved without 

 any striking feature. The carbonic acid gas evolved may be weighed 16 

 and it will be seen that the sum of the weights of the black copper 



lo Gas delivery tubes are usually made of glass tubing as prepared at glass works. It 

 is made of various diameters and thicknesses. If of small diameter and thickness, a glass 

 tube is easily bent by heating in a gas jet or the flame of a spirit lamp, and may also be 

 easily divided at a given point by making a deep scratch with a file and then breaking the 

 tube at this point with a sharp jerk. These properties, together with their impermea- 

 bility, transparency, hardness, and regularity of bore, makes glass tubes most useful in 

 experiments with gases. Naturally they might be replaced by straws, india-rubber, 

 metallic, or other tubes, but these are more difficult to fix on to a vessel, and are not 

 entirely impervious to gases. A glass gas delivery tube may be hermetically fixed into 

 a vessel by fitting it into a perforated cork, which should be soft and free from flaws, and 

 fixing the coi'k into the orifice of the vessel. Sometimes the cork is previously soaked in 

 paraffin, or it is replaced by an india-rubber cork. 



16 Gases, like all other substances, may be weighed, but, owing to their extreme light- 

 ness and the difficulty of dealing with them in large masses, they can only be weighed by 

 very sensitive balances ; that is, in such as, with a considerable load, indicate a very small 

 difference in weight for example, a centigram or milligram with a load of 1,000 grams. 

 In order to weigh a gas, a glass globe furnished with a stop- cock (which must not leak in 

 any part, and therefore must be kept well lubricated) is first of all exhausted of air by an 

 air-pump la Sprengel pump ia the best). The stop-cock is then closed, and the exhausted 

 globe weighed. As the pressure of the atmosphere acts on the walls of the globes, they 

 should be thick. Glass is found to bear the strain of the inequality of the exterior and 

 interior pressures best. If the gas to be weighed is then let into the globe, its weight 

 can be determined from the increase in the weight of the globe. It is necessary, how- 

 ever, that the temperature and pressure of the air about the balance should remain 

 constant for both weighings, as the weight of the globe in air will (according to the laws 

 of hydrostatics) vary with its density. The volume of the air displaced, and its weight, 

 must therefore be determined by observing the temperature, density, and moisture of the 

 atmosphere during the time of experiment. This will be partly explained later, but may be 

 studied more in detail by physics. Owing to the complexity of all these operations, the 

 mass of a gas is usually determined from its volume and density, or the weight of one 

 volume. 



