Sentence:  Although stopping may have served to produce some chromatic sounds, and partially filled up some of the smaller gaps in the <WSD>natural harmonic</WSD> series, the characteristic clearness of trumpet-tone was largely lost when the instrument was thus muted by having the passage through the bell closed by the hand. 
Meaning of the word marked with <WSD> tags:  The series of pitches produced by a wind instrument without modulation by a mute, valves, or keys.