You should avoid soldering together two different species of glass--for example, a tube of ordinary glass with a tube of flint-glass; because these two species of glass experience a different degree of contraction upon cooling, and, if joined together while in a fused state, are so violently pulled from one another as they become cool, that the cohesion of the point of soldering is infallibly overcome, and the tube breaks. You ought also, for a similar reason, to take care not to accumulate a greater mass of glass in one place than in another. If the first operation has not been sufficient to complete the soldering, the tube must be again presented to the flame, and again pushed together at the joint, or drawn asunder, or blown into, according as it may appear to be necessary. In all cases the soldering is not truly solid, but inasmuch as the two masses of glass are well incorporated together, and present a surface continuous in all points. The mineralogical flame (pl. 1, fig. 1, A´ B) is that which is to be employed in preference to the larger flame, when you desire to effect a good joining: it is sufficient to proportion the size of the flame to the object you wish to execute. THE ART OF GLASS-BLOWING. IV.--_Construction of Chemical and Philosophical Instruments._ When a person is well acquainted with the fundamental operations which we have just described, the preparation of the instruments of which we are about to speak can present scarcely any difficulty. Indeed, some of them are so extremely simple, and are so easy of execution, that it is sufficient to cast a glance upon the figures which represent them, to seize at once the method which must be followed in their construction. Of such instruments we shall not stop to give a detailed description, but shall content ourselves with presenting the design. On the other hand, it is of importance to observe that a certain number of instruments are _graduated_ or furnished with pieces, or _mountings_, of which it is not the object of our art to teach the construction, and which demand a more or less extensive knowledge of the sciences. We shall treat of these mountings but summarily, referring the student, for more detailed instructions, to the works on natural philosophy and chemistry, in which these instruments are especially treated of. Our reason for this is, that we do not wish to abandon the plan we had adopted of describing simply the art of glass-blowing. To describe the use and application of philosophical instruments, or to explain the principles on which they act, would be passing quite out of our province.