Page:Proceedings of the Royal Society of London Vol 1.djvu/481

Rh appears to the author to have been the difficulty of making the joinings sufﬁciently ﬁrm to bear the necessary pressure, and at the same time capable of being easily disunited for the purpose of making any new arrangement of the parts ; for in both these respects, as well as in other points, the usual conical joints of glass are very objectionable.

In Mr. Austin’s construction, all the parts are ﬁtted together by plane surfaces, which have the advantage, not only of being easily formed, but of ﬁtting each other in any order of combination in which it may be thought convenient to place them. For this purpose, each part of the apparatus, where there is need of a joint, is originally made by the glass-blower to terminate in a ﬂat, thick, circular plate of glass of a given diameter, that is larger than the neck to which it is attached.

The ﬂat terminations of all the parts being then ground perfectly plane, any two may be applied together, and clamped by a proper system of collars and screws, with the certainty of ﬁtting, without regard to any inequality of the perforations in each, through which the communication between the vessels is thus established.

The clamps employed by the author for connecting his apparatus, consist of two ﬂat circular collars of brass, large enough to admit the circular ﬁat ﬂanches to pass through them, and a pair of mahogany collars of smaller dimensions, so as to ﬁt the necks after they have been sawn in two. The brass collars, having been ﬁrst put upon each vessel. are prevented from returning by the interposition of the wooden collars, and are then screwed together by a suitable number of screws. In rendering these joints perfectly secure, it is necessary to turn all the screws with nearly equal force; and it may sometimes be expedient to moisten the surfaces of glass that are applied to each other when any great degree of condensation is required.

The tube through the piston terminates, at its upper extremity, in a small cup, which is the segment of a sphere, and contains a plane convex lens, ground to the same radius, and ﬁtting so as to perform the ofﬁce of a valve. A similar valve is placed between the barrel and the receiver, and anotheras safety-valve at the top of the receiver, with a spring and screw to regulate the force of condensation that may be applied.

Since the motion of the barrel on the piston is required to be performed with extreme steadiness, it is ﬁrmly attached by strong collars to an iron sliding-bar, guided by a dove—tail groove of brass, ﬁxed to an upright pillar that stands on the platform, to which the ﬁrst reservoir and piston are attached, so as to avoid all possibility of lateral motion that might destroy the apparatus.

The author observes, that if the undermost vessel, instead of being open as a reservoir for air to be condensed in the uppermost, be closed at the bottom, it will be exhausted, so that the same instrument serves the purpose of an air-pump as well as a condenser.