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.
ADAPTERS.--These are tubes of glass of various forms, employed in chemistry to connect together the different pieces constituting an apparatus--as, for example, to join a retort to a receiver during the operation of distillation. You should take care to border the extremities of an adapter; or you may widen them into the form of the mouth of a bottle, when they are to be closed air-tight by corks. Besides this, there is nothing particular to be observed in the preparation of adapters.
APPARATUS FOR BOILING IN VACUO.--Represented by pl. 3, fig. 19. Employ a tube about a quarter of an inch in diameter. Blow two bulbs; give the tube the necessary curvature; fill one of the bulbs with nitric ether; boil the ether to expel the atmospheric air from the apparatus, then seal the opening in the other bulb.
APPARATUS FOR FREEZING IN VACUO. _The Cryophorus._--Take a tube one-third of an inch or rather more in diameter, and pretty thick in the sides. Blow a bulb at each end; the first at the sealed part of the tube, the other at the open point; then give to the tube the curvature represented by pl. 3, fig. 32. Introduce as much water as will half fill one of the bulbs; make the water boil, and draw off the point and seal the apparatus during the ebullition.