Thought Forms is a book, by Annie Besant and C.W. Leadbeater, which is a study on the nature and power of thoughts. 
The authors state that thoughts have two effects: "a radiating vibration and a floating form".
Thought forms are divided into three classes:
•    That which takes the image of the thinker.
•    That which takes the image of some material object.
•    That which takes a form entirely its own, expressing its inherent qualities in the matter which it draws round it.
The effect of music, emotions, and colors on thought forms is also studied in the book.
The effect of the music of Felix Mendelssohn (No. 9 of his "Songs without words"), Charles Gounod (Soldiers Chorus from "Faust") and Richard Wagner (Overture to "The Meistersingers") is studied. The music of Wagner produced a "marvellous mountain-range" on the thought forms.