Written in 1935, Sri Swami Sivananda’s book, classic book, The Science of Pranayama, is available for free download as a PDF and it is also available in hard copy from Integral Yoga Distribution or from the Divine Life Society of Rishikesh  who generously has made the e-book available for free. Here is an excerpt from Chapter one and you can see the complete book here: Science of Pranayama.

Chapter One:  Prana and Pranayama
Pranayama is an exact science. It is the fourth Anga or limb of Ashtanga Yoga. “Tasmin Sati Svasa prasvasayorgativicchedah Pranayamah”—Regulation of breath or the control of Prana is the stoppage of inhalation and exhalation, which follows after securing that steadiness of posture or seat, Asana. Thus is Pranayama defined in Patanjali Yoga Sutras, Chapter II-49.

‘Svasa’ means inspiratory breath and ‘Prasvasa’ is expiratory breath. Breath is external manifestation of Prana, the vital force. Breath like electricity, is gross Prana. Breath is Sthula, gross. Prana is Sukshma, subtle. By exercising control over this breathing you can control the subtle Prana inside. Control of Prana means control of mind. Mind cannot operate without the help of Prana. The vibrations of Prana only produce thoughts in the mind. It is Prana that moves the mind. It is Prana that sets the mind in motion. It is the Sukshma Prana or Psychic Prana that is intimately connected with the mind. This breath represents the important fly-wheel of an engine. Just as the other wheels stop when the driver stops the fly-wheel, so also other organs cease working, when the Yogi stops the breath. If you can control the fly-wheel, you can easily control the other wheels. Likewise, if you can control the external breath, you can easily control the inner vital force, Prana. The process by which the Prana is controlled by regulation of external breath, is termed Pranayama.

Just as a goldsmith removes the impurities of gold by heating it in the hot furnace, by strongly blowing the blow-pipe, so also the Yogic student removes the impurities of the body and the Indriyas by blowing his lungs, i.e., by practising Pranayama. The chief aim of Pranayama is to unite the Prana with the Apana and take the united Pranapana slowly towards the head. The effect or fruit of Pranayama is Udghata or awakening of the sleeping Kundalini.

What is Prana?
“He who knows Prana knows Vedas” is the important declaration of the Srutis. You will find in Vedanta Sutras: “For the same reason, breath is Brahman.” Prana is the sum total of all energy that is manifest in the universe. It is the sum total of all the forces in nature. It is the sum total of all latent forces and powers which are hidden in men and which lie everywhere around us. Heat, light, electricity, magnetism are the manifestations of Prana. All forces, all powers and Prana spring from the fountain or common source, ‘Atman’. All physical forces, all mental forces come under the category ‘Prana’. It is force on every plane of being, from the highest to the lowest. Whatever moves or works or has life, is but an expression or manifestation of Prana. Akasa or ether also is an expression of Prana. The Prana is related to mind and through mind to will, and through will to the individual soul, and through this to the Supreme Being. If you know how to control the little waves of Prana working through the mind, then the secret of subjugating universal Prana will be known to you.

The Yogi who becomes an expert in the knowledge Of this secret, will have no fear from any power, because he has mastery over all the manifestations of powers in the universe. What is commonly known as power of personality is nothing more than the natural capacity of a person to wield his Prana. Some persons are more successful in life, more influential and fascinating than others. It is all due to the power of this Prana. Such people manipulate everyday, unconsciously of course, the same influence which the Yogi uses consciously by the command of his will. There are others who by chance tumble unaware of this Prana and use it for lower purposes under false names. This working of Prana is seen in the systolic and diastolic actions of the heart, when it pumps the blood into arteries in the action of inspiration and expiration during the course of breathing; in the digestion of food; in the excretion of urine and faecal matter; in the manufacture of semen, chyle, chyme, gastric juice, bile, intestinal juice, saliva; in closing and opening of the eyelids, in walking, playing, running, talking, thinking, reasoning, feeling and willing. Prana is the link between the astral and physical body. When the slender thread-link Prana is cut off the astral body separates from the physical body. Death takes place. The Prana that was working in the physical body is withdrawn into the astral body.

This Prana remains in a subtle, motionless, unmanifested, undifferentiated state during the cosmic Pralaya. When the vibration is set up, Prana moves and acts upon Akasa, and brings forth the various forms. The macrocosm (Brahmanda) and microcosm (Pindanda) are combinations of Prana (energy) and Akasa (matter). That which moves the steam-engine of a train and a steamer, that which makes the aeroplanes fly in air, that which causes the motion of the breath in lungs, that which is the very life of this breath itself, is Prana. I believe, you have now a comprehensive understanding of the term Prana about which you had a very vague conception in the beginning.

By controlling the act of breathing you can efficiently control all the various motions in the body and the different nerve-currents that are running through the body. You can easily and quickly control and develop body, mind and soul through breath-control or the control of Prana. It is through Pranayama that you can control your circumstances and character and can consciously harmonise the individual life with the cosmic life. The breath, directed by thought under the control of the will, is a vitalising, regenerating force which you can utilise consciously for self-development; for healing many incurable diseases in your system; for healing others and for other various useful purposes.

It is within your easy reach at every moment of your life. Use it judiciously. Many Yogins of yore, like Sri Jnanadeva, Trailinga Swami, Ramalinga Swami and others, had utilised this breath, this force, the Prana, in a variety of ways. You can also do so, if you practise Pranayama by prescribed breathing exercises. It is Prana that you are breathing rather than the atmospheric air. Inhale slowly and steadily with a concentrated mind. Retain it as long as you can do it comfortably. Then exhale slowly. There should be no strain in any stage of Pranayama. Realise the occult inner life-powers which underlie the breath. Become a Yogi and radiate joy, light and power all around you. Pranavadins or Hatha Yogins consider that Prana Tattva is superior to Manas Tattva, the mind-principle. They say, Prana is present even when the mind is absent during sleep. Hence Prana plays a more vital part than the mind. If you go through the parables in Kaushitaki and Chhandogya Upanishads, when all the Indriyas, mind and Prana fight amongst themselves as to their superiority, you will find that Prana is regarded as the highest of all. Prana is the oldest, for it starts its functioning from the very moment the child is conceived. On the contrary, the organs of hearing, etc., begin to function only when their special abodes, viz., the ears, etc., are formed. Prana is called Jyeshtha and Sreshtha (oldest and best) in Upanishads. It is through the vibrations of psychic Prana that the life of the mind, Sankalpa or thinking is kept up and thought is produced. You see, hear, talk, sense, think, feel, will, know, etc., through the help of Prana and therefore Srutis declare: “Prana is Brahman.”