The concentrated acid is drawn off by means of the platinum siphon tv. The short srm of the siphon reaches fully to the bottom of the still. The long arm passes through the cooler G, into which cold water is constantly introduced by a pipe at the bottom, and flows away from the top of the other end.. The portion of the siphon that traverses the cooler is split into two halves, in order to facilitate the cooling. It is closed or regulated by a tap, sometimes made of gold, but now more generally of platinum. To ensure sufficiently rapid outflow, the tap must be at least 1 ft. 6 in. lower than the bottom of the still. The siptron is set by means of the two little funnel-topped platinum tubes x, so arranged that their tops are higher than the highest part of the siphon.
After dosing the top of the siphon and opening both the feed pipes so much sulphmie acid is poured into one of them as suffices to completely fill the siphon by which the air is expelled at the second pipe. Their stoppers are then replaced. For ease in moving and handling the siphon it is made in several, generally four, pieces fitting one into another, and provided with flanges that may be tightly compressed with screw bolts. A wooden trestle y supports the siphon. The concentra tion is carried on day and night, and by having the furnaces in juxtaposition, as we have shown, one workman can easily conduct the operation.
The surface of the five leaden pans is more than sufficient to concentrate all the acid made by the chamber system wo have given from 1(10° to 150° Tw., at such a gentle heat that very little acid will be distilled with the water. The acid is fed continuously into the first pan, and the flow though the apparatus Is so regulated that the acid leaving the fifth pan is about I33° Tw. when hot, or 150° Tw. at 15° (60° F.). The still is filled to the level of its greatest diameter, and the depth of avid is regularly mslutained at this point. The concentration may be made either continuous or i ntermittent.
With the continuous process the siphon tap is opened to such a degree that the acid which the still is capable of concentrating flows out in a continual stream, and at the same time so much acid is supplied to the pans as will suffice to keep the level in the still at the proper height. The level of acid in the still can always be seen by the float. The strength of the distillate forms a convenient guide as to the strength to which the acid is being concentrated. If the distillate
registers 90° Tw., the concentrated acid will be about 167' Tw., which is the highest grade that can he reached with continuous concentration, on account of the effect constantly exercised by the weak in-flowing acid. With continuous firing something like 34 cwt. of acid can be con centrated with this apparatus.
If it bo important that tho acid shall be more than 167° Tw. the intermittent process must be adopted. In this case the exit tap is closed until the distillate shows about 100° Tw. As soon as this point is reached the supply from the pans is stopped and the fire increased, and a portion of the 'Lek] drawn off as rapidly as possible into carboys. The quantity drawn off must never he so great that the level in the still sinks below the top of the flue n, for the still would be very rapidly destroyed if the fire were allowed to play upon parts which were not covered with acid. Probably about 2 cwt. may be drawn off each time without harm. As soon as this has been done the still is refilled from the fifth pan, whose communication with the fourth pan must at that time be suspended, or a quantity of weak acid may find its way into the still.
When the work is properly managed the still may be tapped fifteen times daily, thus producing about 30 cwt. of acid at 170' Tw. It is natural that the production by this method bo much less than with continuous firing, on account of the suspension of the firing during the time that acid is being drawn from the still.
The most advantageous process is undoubtedly the continuous, by which the smallest amount of fuel is consumed in proportion to the make of acid, and by which the damage to the still is least on account of the almost constant degree at which the temperature is maintained.
The oencentrated acid flows into glass carboys holding from If to 2 cwt. 'I hese are packed in wicker baskets with straw, and closed with stoneware stoppers cemented with molten sulphur. Over these soft clay is put, and then pieces of coarse *coking are tied down round the neck. The distillate is so much purer than ordinary chamber acid that in some cases it may be conveniently used ; in other cases it is run back into tho basin of one of the chambers.
The total loss of acid during tho operation should never exceed 1 to 2 per cent.