As the object is to be thoroughly immersed in the balsam, it is evident that when it has once been covered, so it must remain, unless we again free it by a process hereafter mentioned, which is very troublesome; and on this account there must be nothing whatever in the balsam except the object. The inexperienced may think this an unnecessary caution; but the greatest difficulty he will meet with is to get rid of minute bubbles of air, perhaps invisible to the naked eye, which appear like small globules when under the microscope, and render the slide unsightly, or even worthless. Ten objects out of eleven contain air, or at least are full of minute holes which are necessarily filled with it; so that if they should be immersed in any liquid of thick consistency, these cells of air would be imprisoned, and become _bubbles_. The air, then, must be got rid of, and this is usually accomplished by soaking for some time in turpentine, the period required differing according to the nature of the object. In some cases, the turpentine acts upon the colour, or even removes it altogether, so that it must be watched carefully. Often, however, this is an advantage, as where the structure alone is wanted, the removal of the colouring matter renders it more transparent. There are objects, however, which retain the air with such tenacity that soaking alone will not remove it. If these will bear heat without being injured, they _may_ be boiled in turpentine, or even in balsam, when the air will be partly or totally expelled. But where heat is objectionable, they must be immersed in the turpentine, and so submitted to the action of the air-pump. Even with this aid, sometimes days are required to accomplish it perfectly, during which time the air should be exhausted at intervals of five or six hours, if convenient, and the objects turned over now and then. Sometimes the objects are so minute that it is impossible to submit them to any soaking, and in this case they must be laid upon the slide at once, and the turpentine applied to them there. But it must not be forgotten that there are some few which are much better mounted in such a way that the balsam may thoroughly surround, and yet not _penetrate_, the substance more than necessary. Sections of teeth are amongst these, which will be noticed in another place, and some insects (see Dr. Carpenter) when required to show the “ramifications of the tracheæ.” Having freed the object, then, from these two enemies--dampness and air--we must proceed to mount it.