There are many other conditions which Mr. Booer, after consultation with practical bakers and others, set himself to fulfill, the observance of which lends to the present Blackfriars experiment much of its interesting character. Thus it was observed that, while it is not difficult to build an oven in a given spot, and bake bread in it, this cannot truly be called a _baker's_ oven. By this term must be understood in particular an oven in an ordinary bakehouse, set in the usual style and worked by a man with his living to get by it. Before the problem of extending gas to bakers' ovens could be considered solved, it had to be attacked from this aspect. Mr. Booer, to do him full credit, seems to have early appreciated this fact in all its bearings. He not only saw that it was necessary to save gas, as much as possible, by putting it inside the oven; but he was told that, in order to meet with any general success, the cost of converting an oven to the gas system must be rigidly kept down to about ten or twelve guineas. The latter seems a particularly hard condition, when it is remembered that the only improved baker's oven in practical use at the present day is the steam oven invented by Mr. Perkins, which costs two or three hundred pounds to erect. Mr. Booer also had in mind the necessity that everything possible for a coal oven must likewise be performed by a gas oven; and in this respect he set himself to surpass the costly Perkins oven, which will not bake the common "batch" or household bread, generally the principal article of sale, more especially in populous and poor neighborhoods. The peculiar efficacy of the common coal fire in this respect proceeds from the essential principle of action of a brick oven, which is found simply in the fact that the work is done entirely by heat previously imparted to the tile bottom, roof, and sides of the oven, and thence radiated to the bread. No other kind of heat will bake batch-bread--i.e., loaves packed in contact with one another--which requires to be thoroughly soaked by a radiant heat in a close atmosphere of its own steam. Now, as a coal fire is eminently qualified to impart, by radiation and otherwise, this necessary store of heat to the brickwork, it is plainly a difficulty to effect the same purpose with a fuel which, of itself, can scarcely radiate heat at all. The system of the gas cooking-oven--the utilization of the heat of the combustion products as formed--is clearly inapplicable here; for a different kind of heat is needed, under conditions that would not sustain continuous combustion. Therefore, there is nothing for it but to heat the bottom and sides of the brick oven by the direct contact of powerful gas-flames; thus supplanting the coal fire, but leaving the actual work of baking to be done afterward by stored-up heat in the regular way.