The relative quantity of starch, however, depends not only on the variety of potatoe, but on the nature of the soil, the mode of culture, and the season of the year at which it is prepared. That the same kind of potatoe contains different proportions of starch at different seasons of the year, has been lately shewn by M. Pfaff, who obtained the following quantities from 240 pounds of the same kind of potatoe:*

In August, from 23 to 25 pounds;

In September, from 32 to 38 pounds;
In October, from 32 to 40 pounds;

From November to March, from 38 to 45 pounds;
In April, from 38 to 28 pounds;

In May, from 28 to 20 pounds.

As the proportion of starch is so variable, the manufacturer should always make a preliminary estimation on a small scale of the value of the potatoe to be used in the manufacture, before appropriating a considerable quantity for that purpose. To determine the real value of the potatoe, it is necessary to know, 1°, the amount of water; and 2o, the amount of starch in the dry residue.

The proportion of water may be determined with sufficient accuracy in the following simple manner. The potatoe being well cleaned is cut into very thin

extremely simple means. They are invariably soluble with facility in water; hence the process of washing in cold water suffices for their removal. The inhabitants of Guyana and the Antilles have long been acquainted with the means of separating the prussic acid from the starch deposited from the juice of the manioc. Their method consists in exposing the starch to a moderate heat; when the prussic acid, which is extremely volatile, is entirely dissipated in vapour.

* According to M. Girardin, frosted potatoes afford as much starch as those which are unfrosted.

slices, which are weighed and then placed side by side on a plate, and covered with a piece of bibulous paper. The slices are then exposed to a moderate heat (about 110° or 120° Fahr.) in a drying stove or other convenient place. The completion of the drying is known by the weight of the slices re maining constant after intervals of a quarter of an hour or so. They are then hard and brittle, and the loss on their original weight gives the proportion of water required.

The amount of starch is estimated by a process quite analogous to that practised for its extraction on the large scale. A fresh quantity of washed potatoes is reduced to a fine pulp, with the assistance of a rasp, weighed, and placed on a fine sieve of horsehair or metallic gauze. A small stream of water is allowed to fall on the centre of the sieve, the pulp being at the same time briskly agitated between the hands. This operation is continued so long as the water passes through the sieve in a milky state. The starch liberated by the rupture of the vegetable cells is carried by the water through the sieve, and is deposited on the standing of the milky liquid. When fully subsided, the supernatant liquid is poured off, the starch agitated with a fresh quantity of water, and again allowed to subside: the decantation and affusion of fresh water having been repeated two or three times, the starch may be drained, carefully dried in a stove, and weighed.

As the starch is not distributed over the tuber equally, in making such an analysis as this the part operated on is not a matter of indifference. The starch is contained in largest quantity towards the

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exterior; sometimes, especially in large potatoes, the centre is almost transparent, from containing nothing besides cellular tissue and water. Immediately under the epidermis, however, is situated a thin layer or zone of cellular tissue devoid of starch; the tissue immediately underneath this contains the starch in large quantity, and the proportion decreases gradually towards the centre (M. Payen). Hence, to obtain a sample representing the true composition of the tuber, the slices should be cut in such a manner as to contain a fair proportion of each of the layers.

The process of preparing potatoe starch on the large scale, about to be described, is that followed in the principal establishments in France, where this manufacture is conducted on a more extensive scale than in England. I am chiefly indebted for the description of it to the Cours de Chimie Organique of M. Payen (1842).

In keeping potatoes in store before they are required for use, care should be taken to preserve them from the influence of any agents which are favourable to the decay or to the germination of the tuber. Potatoes have been preserved for a whole year in an ice-house at the temperature of the freezing point of water without any sensible loss in their proportion of starch. It was common formerly to keep them in heaps in subterranean cellars, but this plan is not approved of now, as potatoes thus kept are very subject to a fermentation excited originally by bruises on the surface, and favoured by the retention of the heat thereby developed. The common method of preserving potatoes followed at present is to imbed them in large shallow ditches

(silos), dug, if possible, in a sandy soil. They should be situated near the starch-works, and be protected from the access of air by a thatched straw or other covering. To avoid or lessen the fermentation referred to, the ditch should be provided with small apertures to serve as ventilating chimneys, by which the heated air and gases may escape. These apertures may be made by planting some dry chat-wood in the mass.

The form of apparatus generally employed at present in the manufacture may be understood with the assistance of the design in the following page. It consists essentially of three parts; a washing cylinder for the potatoe, a rasping machine, and a sieve.

1. An indispensable preliminary operation in the manufacture is the soaking of the potatoes for about six hours in water, the object of which is to soften the epidermis, and thus facilitate its removal. They are then transferred through the hopper a, to a cylindrical cage b, and washed by revolving the cage in a trough of water c, a jet of water flowing at the same time from the cistern d, through the pipe e. All the earthy matter, and a great portion of the skin itself, are separated by the friction of the potatoes against each other. In one French manufactory the potatoes are washed in two cylinders successively.

2. The next operation consists in the reduction of the potatoes to a fine pulp. This is a process of much importance, as the quantity of the product of starch depends in a great measure on the amount of disintegration of the cellular tissue.