The use of mercuric oxid in this operation greatly shortens the time necessary for digestion, which is rarely over an hour and a half in case of substances most difficult to oxidize, and is more commonly less than an hour. In most cases the use of potassium permanganate is quite unnecessary, but it is believed that in exceptional cases it is required for complete oxidation, and in view of the uncertainty it is always used. The potassium sulfid removes all the mercury from the solution, and so prevents the formation of mercurammonium compounds which are not completely decomposed by soda solution. The addition of zinc gives rise to an evolution of hydrogen and prevents violent bumping. Previous to use, the reagents should be tested by a blank experiment with sugar, which will partially reduce any nitrates that are present, which might otherwise escape notice. [Illustration: FIGURE 13. DISTILLING APPARATUS.] =185. The Distillation Apparatus in Use in the Laboratory of the Department of Agriculture.=—In this laboratory the distilling apparatus is arranged as shown in Figure 13. The flasks are the same as are used in the digestion. They are connected to the block tin condensers by the bulb and rubber tubes, shown hanging on the projecting ends of the block tin condensers on the right and left of the figure. The condensers are contained in a trough through which cold water flows during the distillation. The bulb above the flask carries an emergent tube which extends to near the center of the bulb and is bent laterally to avoid any danger of carrying over any alkali that may be projected into the bulb during boiling. The boiling is continued usually for nearly an hour or until bumping begins. The table on which the apparatus is placed is so arranged as to permit of easy access on all sides. The standard acid is held in erlenmeyers placed on wooden blocks so that the end of the condenser which is a drawn-out glass tube, dips beneath the surface of the acid. =186. Patrick’s Distilling Flask.=—To avoid the expense and annoyance attending the breaking of the distilling flasks Patrick has proposed to make them of copper.[154] The size, about half a liter, made for the evolution of oxygen for experimental purposes, may be used. A little excess of potassium sulfid is used to make up for any of it which might be consumed by the copper. About twenty-five cubic centimeters of this solution are recommended. No zinc or pumice stone is required to prevent bumping and the distillation may be finished within thirty minutes, thus securing a saving of time. There will doubtless be a slight corrosion of the flasks by the sulfid employed but where the gunning oxidation process is practiced this danger would be avoided.