ADVERTISEMENT.

IN presenting to the public the concluding volume of the SCIENTIFIC DIALOGUES, the Author cannot but acknowledge, with sentiments of gratitude, the favourable reception which the former parts of the work have experienced. He trusts that the several subjects comprised in this last volume, will have an equal claim to the candour of those who are engaged in the arduous but honourable employment of education.

It will be seen that it was quite impossible to include in the three volumes the introduction to chymistry, as it was originally intended. This branch of science is become so very interesting, and the study of it so

general, that it would have been unpardonable to have devoted only a part of a volume to the discussion of it: the Author has, therefore, at the suggestion and desire of many friends, who have given their approbation to the Scientific Dialogues, undertaken to furnish a separate work on this subject, in two volumes, similar in size to this.

OPTICS.

little satisfaction to be told that this deception was caused by the different degrees of refraction which takes place in water and in air.

James. We do not know what you mean by the word refraction.

Tutor. It will therefore be right to proceed with caution; refraction is a term frequently used in the science of optics, and this science depends wholly on light.\

James. What is light?

Tutor. It would, perhaps, be difficult to give a direct answer to your question, because we know nothing of the nature of light, but by the effects which it produces. In reasoning, however, on this subject, it is generally admitted that light consists of inconceivably small particles; which are projected, or thrown off from a luminous body with great velocity, in all directions.

Charles. But how is it known that light is composed of small particles?

Tutor. There is no proof indeed that light is material, or composed of particles of matter, and therefore I said it was generally,

CONVERSATION I.

INTRODUCTION.

Of Light-The Smallness of its Particles-Their Ve locity-They move only in straight Lines.

CHARLES. When we were on the sea, you told us that you would explain the reason why the oar, which was straight when it lay in the boat, appeared crooked as soon as it was put into the water.

Tutor. I did: but it requires some previous knowledge before you can comprehend the subject. It would afford you but