The residue of chief importance is the hydroxid or hydrate of alumina, which is the form in which the alumina itself should appear when a complete reaction like that defined above takes place. When the hydroxid of alumina is dried and especially when ignited it is converted into an oxid of alumina which is highly insoluble in water and only slightly soluble in a very dilute acid solution. The claim is made by the manufacturer of alum powders that the aluminium residue which is formed is insoluble in the digestive juices and therefore cannot produce any effect usually ascribed to the soluble salts of aluminium. It is important that the conditions which are found in the baking of a loaf are such as to produce this highly desirable result. The temperature of the interior of the loaf during baking does not rise much above that of boiling water, although the exterior temperature, which is sufficient to produce the browning of the crust, is very much above that temperature. It is evident that as long as any considerable proportion of water remains in the loaf it will be difficult to raise the interior of the loaf to the temperature just mentioned, and if this were done the caramelization would take place throughout the whole loaf. Unfortunately, from a scientific point of view the investigation of this subject has not been always undertaken under conditions which are wholly beyond criticism. Many of the investigations have been in the interest of rival baking powder companies, and it is very desirable that this matter should be undertaken in a wholly unbiased way and conducted in such a manner as to lead to results which all will accept. Chemical and physiological investigations, which have even as a remote object the promotion of the sale of one compound and the repression of the sale of another, lose at the outset much of that claim upon the public confidence which such investigations made from a purely scientific point of view should have. _General Statement._--In respect of the use of chemical leavening agents in general it may be said that they introduce an extraneous product into the bread which is not likely to promote the health and which, therefore, on general principles should be excluded. On the other hand, large experience has shown that the consumption of bread made by these leavening agents does not produce any general effect upon the public health which is noticeable. This, it is understood, is not any valid argument in favor of the process. It must also be acknowledged that a fermentation of a bread with yeast also introduces extraneous matter into the food, viz., alcohol and congeneric products of fermentation, and hence this process may be open to a certain extent to the same objection as the one above. It is too early yet to formulate definite principles either of inclusion or exclusion of these products, and the purpose of this manual is secured when the general character and effects thereof are briefly outlined.