(a) The air-bath ordinarily used in chemical laboratories for drying precipitates, for making determinations of water by loss, and for similar purposes, is usually a rather expensive piece of apparatus. The iron or copper closet, with its door, tubulure for thermometer, shelves, stand, etc, works no more satisfactorily because of its somewhat elaborate or difficult construction. In Fig. 17 A, is shown a simple substitute for this apparatus, that as regards simplicity cannot well be excelled, while its other good features certainly operate to commend it. It consists of an inverted flower-pot sustained upon an ordinary tin pan or sand bath, the whole being carried by a tripod or retort stand. The aperture at the top serves to receive a perforated cork, through which a thermometer is passed. An ordinary Bunsen burner is used to heat it. As the sand bath directly oyer the burner becomes very hot, it is advisable to invert a second smaller sand bath within the first, as shown in B. This prevents too direct a radiation of heat from the hot metal. Upon this the little stand or bent triangle supporting the crucible or watch glass containing the substance to be heated may be placed.
The thermometer should be thrust down through the cork until its bulb is near the substance to be dried, so as to obtain a correct indication of the temperature at that point. The entire arrangement is shown in external view in A.
To place a vessel in it or to remove one, the flower-pot is lifted off the sand baths. It will, be observed that its porous nature provides a species of ventilation, while its composition assures it against corrosion. It even protects the plates "below to a considerable extent, as drops of water or other fluid cannot run down its sides as it cools.
But convenient as it is in the role of air bath for simple drying operations, it will be found more so where drying tubes or retorts have to be manipulated at constant temperature. The flowerpot can be perforated at any place, and holes of any size or shape can be drilled and cut through it with an old knife, file, or other implement. Thus in C it is shown in use for drying a substance at constant temperature in a straight drying tube. The holes to receive this tube can be drilled in a few minutes. The arrangement as shown is of the simplest kind, but if the usual bath was used, it would require a special tubulation to be introduced or contrived for the tube to pass through. Flower-pots cost so little that there need be no hesitation in preparing them for special uses.
In D a U tube is shown as being heated, while in E a retort occupies the bath, and is in use for fractional distillation or other operation requiring a constant temperature, In all cases it is better to use the second bath inverted within the chamber. It conduces greatly to the maintenance of an even temperature throughout the whole space. A hint may also be taken from the heavy drying plate formerly perhaps more used than at present. If for the light metal pans a heavy plate } in. or more in thickness is substituted, the temperature will not be subject to as rapid variations, and less difficulty will be experienced in keeping a constant temperature. The tray furnished with the neit large size of pot may be used instead of the sand bath upon which to rest the inverted flower-pot. This gives an absolutely non-corrodible construction. 5
When the bath is in use for drying substances, its top, which is at a rather low heat, affords an excellent place of drying precipitates wrapt in their filter papers. It acta in two way*. It is generally just hot enough to dry them with reasonable quickness without danger of spurting, and it also acts by capillarity to absorb the water directly. It represents in the last respect the porous tile or blotting paper - appliances too little appreciated by chemists here. It must be remembered that the drying of a precipitate by evaporation leaves all the impurities of the wash water concentrated therein, while capillary absorption removes a great part of both wash water and its impurities, thus conducing to the accuracy of the work. (T O'Conor Sloane, Ph.D.)
(b) Bearing in mind the universal and indispensable utility of the air-bath to the chemical analyst, one might expect to find much variety and perfection in the design and construction of such an important piece of apparatus. Strange to say, this is not so. I very much doubt if there is to be found any piece of chemical apparatus in the chemist's laboratory that has received less attention, or stauds more in need of it.
This fact was forced upon my attention by the great difficulty found in bringing certain hygroscopic substances to a constant weight, and I soon discovered that the attempt was hopeless with the ordinary instrument, the reasons for which were not far to seek. In the first place, in the ordinary bath it is impossible to maintain a uniform temperature throughout the whole of the drying chamber, for, even with the help of a thermostat, though it may be regulated with accuracy for some one special portion, other parts will in all probability be found to be several, indeed many, degrees hotter or colder according to the circumstance, so that to dry a substance, say at 100° C, the bath, though regulated to stand at this point for the spot where the thermometer is placed, is no assurance that the thing to be dried is exposed to the temperature desired, and if the object is of any considerable bulk the probability is that one portion of it may extend into a region that is much below a hundred, and another into a place that is much above a hundred.
Naturally the greatest heat is found near the floor, where the thermometer is never placed, and the least heat at the sides and in the corners, where radiation and stagnation mostly take place; but more especially it is cold in the line of draughts that proceed from the chinks of the door and through the primitive contrivance that is usually provided for ventilation. In these parts the temperature may be very little above that of the externa] atmosphere. No wonder then that the complete drying of delicate hygroscopic organic substances is found to be so difficult, for before the colder . part can be made to give up the last trace of moisture that clings so tenaciously to it, the hotter is over dried and stands the risk of being charred.