For many reasons, such as capacity for isolation when being recharged or repaired, it is highly desirable that each item in an acetylene plant shall be separated, or capable of separation, from its neighbours; and this observation applies with great force to the holder and the decomposing vessel of the generator. In all large plants each vessel should be fitted with a stopcock at its inlet and, if necessary, one at its outlet, being provided also with a by-pass so that it can be thrown out of action without interfering with the rest of the installation. In the best practice the more important vessels, such as the purifiers, will be in duplicate, so that unpurified gas need not be passed into the service while a solitary purifier is being charged afresh. In smaller plants, where less skilled labour will probably be bestowed on the apparatus, and where hand-worked cocks are likely to be neglected or misused, some more, automatic arrangement for isolating each item is desirable. There are two automatic devices which may be employed for the purposes in view, the non-return valve and the water-seal. The non-return valve is simply a mushroom or ball valve without handle, lifted off its seat by gas passing from underneath whenever the pressure of the gas exceeds the weight of the valve, but falling back on to its seat and closing the pipe when the pressure decreases or when pressure above is greater than that below. The apparatus works perfectly with a clean gas or liquid which is not corrosive; but having regard to the possible presence of tarry products, lime dust, or sludge, condensed water loaded with soluble impurities, &c., in the acetylene, a non-return valve is not the best device to adopt, for both it and the hand-worked cock or screw- down valve are liable to stick and give trouble. The best arrangement in all respects, especially between the generator and the holder, is a water-seal. A water-seal in made by leading the mouth of a pipe delivering gas under the level of water in a suitable receptacle, so that the issuing gas has to bubble through the liquid. Gas cannot pass backwards through the pipe until it has first driven so much liquid before it that the level in the seal has fallen below the pipe's mouth; and if the end of the pipe is vertical more pressure than can possibly be produced in the apparatus is necessary to effect this. Omitting the side tube _b_, one variety of water-seal is shown at D in Fig. 7 on page 103. The water being at the level _l_, gas enters at _a_ and bubbles through it, escaping from the apparatus at _c_. It cannot return from _c_ to _a_ without driving the water out of the vessel till its level falls from _f_ to _g_; and since the area of the vessel is much greater than that of the pipe, so great a fall in the vessel would involve a far greater rise in _a_. It is clear that such a device, besides acting as a non-return valve, also fulfils two