Now, in order to comprehend what the author really means, it is necessary to do here what generally seems to him to be out of place, but for which there is urgent cause in the case of this book, namely, to introduce certain personal data. Of course, nothing will be said in this connection but what bears upon the author’s decision to write this book. What is said in it could not be justified if it bore merely a personal character. A book of this kind is bound to proffer views to which any person may attain, and these views must be presented in such a way as to suggest no shade of the personal element, that is, as far as such a thing is possible. It is therefore not in this sense that the personal note is sounded. It is only intended to explain how it was possible for the author to understand the above characterized opinions concerning his presentations, and yet was able to write this book. It is true there is one method which would have made the introduction of the personal element unnecessary—this would have been to specify in detail all those particulars which would show that the statements here made are in agreement with the progress of modern science. This course would, however, have necessitated the writing of many volumes, and as such a task is at present out of the question, the writer feels it necessary to state the personal reasons which he believes justify him in thinking such an agreement thoroughly possible and satisfactory. Were he not in a position to make the following explanations, he would most certainly never have gone so far as to publish such statements as those referring to heat processes. Some thirty years ago the author had the opportunity of studying physics in its various branches. At that time the central point of interest in the sphere of heat phenomena was the promulgation of the so-called “Mechanical Theory of Heat,” and it happened that this theory so particularly engrossed his attention that the historical development of the various interpretations associated with the names of Julius Robert Mayer, Helmholtz, Joule, Clausius, and others, formed the subject of his continuous study. During that period of concentrated work he laid those foundations which have enabled him to follow all the actual advances since made with regard to the theory of physical heat, without experiencing any difficulty in penetrating into what science is achieving in this department. Had he been obliged to confess himself unable to do this, the writer would have had good reason for leaving unsaid and unwritten much that has been brought forward in this book.