1. That the sulphate of copper (blue vitriol) cannot be used in making bread for the purpose of rendering it lighter or whiter, because it prevents its rising, and gives it a disagreeable colour;
2. That by reducing it to ashes, and employing suitable means, almost all the salt of copper added to the bread may be collected again.
We should exceed the bounds of this Article if we were to re-produce the numerous reports on the bread or flour submitted to the analysis of the council, and especially on the bread and flour destined for the use of prisons, on mixed flour, and on the quality of bread prepared from flour mixed with starch. The council after examining this bread remarked, that it was not disagreeable to the taste, nor liable to injure the health. However, they were not able to pronounce on its nutritive qualities. It has therefore been recommended that if bread made of flour so mixed was offered for sale, it should have a peculiar form, in order that the public should know what is the nature of the food which is sold to them. The same conclusions have been come to with respect to the sale of bread made of flour mixed with a seventh of the flour of rice. This bread is, according to the council, savoury and it keeps well, and does not become hard so soon as the bread prepared in the ordinary way. As to its nutritive qualities, the council cannot determine on this particular, the question being one of those which, in the actual state of science, is the most difficult, and which can only be solved by a prolonged use of the bread. To complete the series of reports on all that concerns this species of food, we must speak of the leaden reservoirs made use of by bakers. It was of moment to know whether the employment by the bakers of Paris of leaden reservoirs to keep the water used in making bread could give rise to accidents; whether these reservoirs should be prohibited, or whether they might be allowed with certain modifications?
The council have studied this important question, which is become among chemists an object of controversy. Some have affirmed that the water gets charged with oxide of lead by remaining in reservoirs formed of this metal. Other chemists, of no less repute, and among others Guyton de Morveaux, have established, on the contrary, that the presence of a neutral salt, like sulphate, nitrate or muriate, in whatever quantity, as 000·2, suffices to prevent the water from dissolving the lead; and they explain in this way the use that is made, without any ill effects, of the water of the Seine, and of wells, preserved in leaden vessels, with or without exposure to the air.
This diversity of opinion rendered necessary numerous experiments, which have been made with the greatest exactness by a commission of the council. It results from these experiments:—
1. That distilled water put into a reservoir gives rise at the end of some minutes to the formation of a salt of white-lead, but that this salt does not dissolve in the water, and is precipitated, on the contrary, to the bottom of the reservoir.
2. That the waters of the Seine, and of wells, placed in leaden reservoirs, have given rise, at the point of contact of the water and air, to the formation of a white saline matter, which does not dissolve in water but is precipitated to the bottom of the vessel.
3. That the gaseous Seltz water acts the same on the leaden reservoirs as the water of the Seine, and of wells. Before affirming what precedes, the commission left some water for several weeks in four leaden reservoirs. The liquid was almost entirely evaporated, and the remainder of the water, when filtered, showed no trace of lead on the application of the most delicate tests, such as the chromate of potash, hydrosulphuric acid, and hydriodate of potash.
Water which had remained in a bucket, spread over at the moment, and throughout its whole extent, with a saline matter composed of carbonate of lead and of lime, of sulphate of lime and organic substances, did not leave the slightest trace of lead by the action on the water of the most powerful tests.
In consequence of these experiments, the council pronounced a formal opinion, that the bakers might be permitted the use of leaden reservoirs on condition that they put a cock three inches from the bottom of the reservoir, in order that if the insoluble carbonate formed it might be deposited in the water below the cock, and with the further condition that the reservoir should be cleaned once a-month. For greater security, the council thought that it should be required of bakers to cover over the lead which lines these reservoirs with a thin coat of wax, which would prevent the contact of the water with the metal, and stop the formation of the insoluble carbonate of lead. To apply this wax it is only necessary to heat slightly the lead, and rub it rapidly and several times with a piece of wool done over with wax.